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Integrating PayPal Payments
into E-Commerce Applications with ASP.NET

By Rick Strahl


Last Update:

February 10, 2009


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E-Commerce applications require mechanisms for payment. Although more commonly than not e-Commerce sites will use full credit card processing gateways, giving PayPal as an additional option of payment provides additional payment options for your customers, especially those that donít want to pay by credit card over the Internet. If you run a Web shop that uses direct credit card processing and want to integrate PayPal seamlessly, youíll find that using PayPal as a processing service is not as straight forward as using a payment gateway. In this article I describe how you can minimize the external PayPal interaction and work the PayPal payment into the order processing workflow to provide a seamless interface using ASP.NET and C#. However, the principles used here should be applicable to any Web programming environment.


Payment processing on the Web is a big market and there a myriad of services and providers that allow you to process credit cards and bank cards over Internet connections. Most of these services are using credit card processing that automates the process of credit card validation. These providers usually provider either HTML or API based interfaces to process credit cards. HTML based interfaces provide you with a URL your application can redirect to on the providerís site, which then handles the actual payment processing. API based interfaces allow your application talk directly to the providerís services so that the payment processing can be integrated into your own application.


If youíre running your own E-Commerce applications you generally will want to use an API so the payment processing can be directly integrated into your own application. This makes sure the user of your app sees a consistent Web interface Ė as far as the user is concerned they are never leaving your site, but rather your Web backend is making the remote calls against the payment processing service and simply returning the success or failure of the request. HTML based payment interfaces generally look unprofessional as the user is whisked off to some other site for payment processing. Using external HTML processing often also requires you to handle inventory and order management through the payment provider which can be hassle especially if your e-commerce app already handles this. Still HTML based interface are popular because they can often be driven from non-data driven site and thus mitigate all the order management to the processing providerís servers which can be beneficial for small sites.



Why PayPal?

On my own Web Siteís West Wind Web Store I use API based processing with Authorize.NET to perform most payment processing.  This works great is fast and provides an integrated mechanism that provides seamless integration. We sell our software products online and from time to time there are those customers that are still squeamish about using their credit cards online Ė often customers from Europe Ė that would rather use an alternate payment mechanism. So a while back I looked at integrating PayPal into my store software.


PayPal may not be the first choice for order processing in a custom e-Commerce application, but it does provide another payment option for your customers. Some customers (at least in my business) are notoriously difficult in finding the Ďrightí payment mechanism and PayPal has been the answer for me in a number of cases, which is why I eventually sat down and integrated it into my Web Store application.

I've actually been surprised how many orders I get in my store through PayPal. When I originally hooked this up a few months back I thought of it more of a novelty and another bullet item for the store software, but there are quite a few people left in this world who don't trust the Internet (or the vendors for that matter) with their credit cards. PayPal is nice in that respect in that they provide the ability to keep the credit card from any vendor at all - now you just have to worry that PayPal stays safe and honest.

On the merchant front too I've found a lot of people actually using the PayPal functionality as their main payment mechanism instead of merchant services. A lot of small operations or personal sites don't want to deal with merchant services it seems. I can't blame them - most merchant providers or at least the 'resellers' are crooks trying to make a quick buck of the unsuspecting vendor. Getting set up also takes some amount of paperwork and often can take weeks. Luckily that seems to be changing somewhat with better and more consistent gateway services appearing with reasonable prices (my experience with MerchantPlus and Authorize.Net has been a real joy actually). Still, with PayPal you hit the ground running as soon as you have configured your account and provided a bank account.

On the downside, PayPal is a purely Web based interface. If you need to integrate payment processing into a non-Browser based application, PayPal is not an option since PayPal works exclusively through the Web browser interface. So if your business also takes phone orders and has an offline POS system, PayPal will be a hassle as you'd have to enter orders on the Web. PayPal also has rather high sales percentages that are generally much higher than merchant accounts (unless you are super high risk). Be sure you do the math. A little time up front may save you a lot of money in the long run - percentages are what often make or break your sales margins.

PayPal acts as a middle man between the buyer and seller so that the seller never has to expose his payment information to the seller directly. Instead the buyer registers his credit card or bank account with PayPal and maps his account to an email address and then uses PayPal to make purchases. The advantage here is that the seller never gets the buyers payment information and therefore canít abuse it (accidentally or otherwise) in any way..


So what do you need to accept payments? The nice thing about PayPal is that itís easy to sign up to accept payments. If you already have a PayPal account you use for buying things online, then you are already set up to also receive payments. All you need to have a bank account so payments can be deposited into it. Thatís it! 

How PayPal works

PayPal is not a payment gateway and they donít provide a direct API interface to their service, except for very large/high volume customers. Rather, like the HTML based services I mentioned earlier, the buyer has to go to the PayPal site, login and accept the payment request. However, this process can be automated to some extent by configuring PayPal to return to specific URLs in case of success or failure and providing a callback confirmation Url that allows you to verify that  PayPal processed a payment on your behalf. This configuration is done via POST data or QueryStrings passed to the PayPal pages.


In my Web Store I want PayPal to process only my final order amount. PayPal supports a number of order and inventory management features, but in an e-Commerce site scenario such as mine, all of this handled by my own Web application Ė all I want PayPal to do is process the final order amount.

As you might expect from this description this process is fairly involved if you want to completely integrate the payment into the application. The process goes like this:


  1. User picks items in our store Web site
  2. User fills out order info and goes to our storeís order page
  3. We provide the order total
  4. User accepts the order of multiple items and final amount
  5. User selects PayPal as payment
  6. User gets redirected to the PayPal site
  7. PayPal site takes user through the payment process
  8. On Success PayPal redirects back to our order confirmation page
  9. On Failure PayPal redirects back to our order form (redisplays order info)
  10. Finally we need to confirm to PayPal that we want to process this order
    and PayPal confirms to us that the order went through.


Figure 1 shows the general flow of the process.


Figure 1: The process of integrating a PayPal payment involves several pages and HTTP POST backs.


This process looks pretty complicated and while there is quite a bit of back and forth, itís a pretty straight forward once you understand the flow. You need 2 pages on your site to make this work right:


Order Page

This form is the jumping off and return point and in my example itís called OrderForm.aspx. This page needs some internal logic to redirect to PayPal if PayPal is selected as payment. Iíll provide you with a helper class that makes this a snap by simply setting a few properties. The same OrderForm.aspx also serves as the return page from PayPal on completion. When you redirect to PayPal, you provide URLs for success and cancelled operations, and these URLs point back to this page with special querystring parameters (for example: OrderForm.aspx?PayPal=Success). I like to use a single page here to keep the PayPal specific logic in one place, but you can also use separate pages.


Unlike credit card processing itís somewhat difficult to keep PayPal logic inside of a business object, since this interaction relies on the HTTP mechanisms and Page callback, so some of the routing logic ends up embedded as part of your Web UI layer (ASPX code behind). The helper class abstracts most of the internal knowledge but some Ďoperationalí code still squeaks into the ASP.NET page.


For my Web Store application it also makes more sense to return to this page in order to complete the order process. The OrderForm can simply pick up right where it left off before redirecting to PayPal. In short, it makes PayPal integration easy with minimal changes to the existing processing.


Itís important that you understand that the return from PayPal does not guarantee that the order went through! Itís easy to spoof the return URL you sent to PayPal since itís visible on the querystring (or if you POST in the POST buffer). Therefore a user could simply type the Confirmation Url in directly and you should not confirm the order at this point. You can manually check for orders on the PayPal site or wait for PayPalís confirmation emails etc. all of which let you know for sure that the order was processed in a Ďmanualí way.

Instant Payment Notification Callback Page

To automate this process, PayPal can optionally ping you back at another URL with order completion information. This is where the second IPN Confirmation page shown in Figure 1 comes in. It uses a mechanism called Instant Payment Notification (IPN) which is essentially a Web based callback mechanism that calls a pre-configured URL on your site. IPN must be enabled on the PayPal site and when enabled IPN sends a confirmation to you at this URL after the order was processed. PayPal then expects a return from you within a certain timeframe (a few minutes) and return a response to you to confirm your that the customer has paid. To do this you have to POST the data back to PayPal by echoing back all the form data that PayPal sends to you.


IPN is optional, but itís a requirement if you need to confirm your orders immediately with your customers. For example on the West Wind site we confirm new software purchases by sending an email with download URLs to get the software almost immediately. For credit card transactions we send the email immediately as part of the Confirmation.aspx page. When the order is verified and complete, email confirmations are sent. For PayPal orders however we cannot do this through the Confirmation page, so rather the confirmation page just displays the completion notice. The IPN return page then is then used to send the final email confirmations for emailing the download links.



A walk through of the process

Letís take look at the Web Store and the process you go through when you pay with PayPal using the mechanism described above. The process starts out on the Order Form of the main Web Store application. The shopping cart is managed by my e-Commerce application and the Order Form is the final step in the order process. A previous form has already collected customer information so the page shown in Figure 2  only collects the payment information plus any informational pieces from the customer.


Figure 2: The first step in the store is to make order arrangements as usual. In this case customer info was captured on a previous form and this page confirms the order totals. The $627.90 total will get send to PayPal.


For PayPal processing the customer selects the PayPal payment method and clicks on Place Your Order to proceed. For normal credit card processing this page would go ahead and contact the CC processor directly then confirm the order. This process is linear and happens all on a single page request.


But for PayPal, weíll need to intercept the request, go off to PayPal and return back to this page after PayPal has taken the customerís money. The OrderForm.aspx page intercepts the straight order process and instead redirects to PayPal using the code like this:


// *** Handle PayPal Processing seperately from ProcessCard() since it requires

// *** passing off to another page on the PayPal Site.

// *** This request will return to this page Cancel or Success querystring

if (this.txtCCType.Text == "PP" && !this.PayPalReturnRequest)



The if block checks for PayPal Payments and a flag that makes sure that we actually want to redirect rather than handle a return from a PayPal page. If so, HandlePayPalRedirection is called and performs the redirection by building up a URL that gets sent to PayPal. The method also stores several of the current formís input values to a Session object Ė this is so that when we return from PayPal to this page the data the user just typed in can be filled back in. In other words weíre saving the relevant state of this form, and weíll restore it when we return. The redirection process starts off like this:


// Redirects the current request to the PayPal site by passing a querystring.

/// PayPal then should return to this page with ?PayPal=Cancel or ?PayPal=Success

/// This routine stores all the form vars so they can be restored later

/// </summary>

private void HandlePayPalRedirection()


      DataRowContainers.wws_invoiceRow rowInv = this.Invoice.GetTypedDataRow();


      // *** Set a flag so we know we redirected to minimize spoofing

      Session["PayPal_Redirected"] = "True";


      // *** Save the Notes and Types so we can restore them later

      Session["PayPal_Notes"] = this.txtNotes.Text;

      Session["PayPal_HeardFrom"] = this.txtHeardFrom.Text;

      Session["PayPal_ToolUsed"] = this.txtToolUsed.Text;


      PayPalHelper PayPal = new PayPalHelper();

      PayPal.PayPalBaseUrl = App.Configuration.CCPayPalUrl;

      PayPal.AccountEmail = App.Configuration.CCPayPalEmail;


      PayPal.LogoUrl = "https://www.west-wind.com/images/wwtoollogo_text.gif";


      PayPal.Amount = rowInv.Invtotal;


      PayPal.InvoiceNo = rowInv.Invno;

      PayPal.BuyerEmail = this.Invoice.Customer.GetTypedDataRow().Email;


      PayPal.ItemName = App.Configuration.StoreName + " Invoice #" + rowInv.Invno;


      PayPal.SuccessUrl =Request.Url + "?PayPal=Success";

      PayPal.CancelUrl = Request.Url + "?PayPal=Cancel";







Notice the use of the PayPalHelper class which Iíll describe a little later. This reusable class provides a simple interface to perform some parsing tasks. In this case, the GetSubmitUrl() method is used to create a string that returns a fully qualified PayPal URL that takes your customer to the PayPal site with your account selected.


The URL generated looks something like this:


https://www.paypal.com/cgi-bin/webscr?cmd=_xclick&business=YourEmail@YourCompany.com &email=CustomerEmail@Company.com &amount=310.96&image_url=https://www.west-wind.com/images/wwtoollogo_text.gif&item_name=West Wind Web Store Invoice #8d5c9631&invoice=8d5c9631&return=https://www.west-wind.com/wwWebStore/OrderForm.aspx?PayPal=Success&cancel_return=https://www.west-wind.com/wwWebStore/OrderForm.aspx?PayPal=Cancel


Actually the content needs to be UrlEncoded, which is also handled by the above class. The GetSubmitUrl() method is pretty simple Ė it does little more than take the various property values set on the class and builds the query string from it.


public string GetSubmitUrl()


      StringBuilder url = new StringBuilder();


      url.Append( this.PayPalBaseUrl + "cmd=_xclick&business="+

            HttpUtility.UrlEncode( AccountEmail ) );


      if( BuyerEmail != null && BuyerEmail != "" )

            url.AppendFormat( "&email={0}", HttpUtility.UrlEncode( BuyerEmail ) );


      if (Amount != 0.00M)

            url.AppendFormat("&amount={0:f2}", Amount);


      if( LogoUrl != null && LogoUrl != "" )

            url.AppendFormat( "&image_url={0}", HttpUtility.UrlEncode( LogoUrl ) );


      if( ItemName != null && ItemName != "" )

            url.AppendFormat( "&item_name={0}", HttpUtility.UrlEncode( ItemName ) );


      if( InvoiceNo  != null && InvoiceNo != "" )

            url.AppendFormat( "&invoice={0}", HttpUtility.UrlEncode( InvoiceNo ) );


      if( SuccessUrl != null && SuccessUrl != "" )

            url.AppendFormat( "&return={0}", HttpUtility.UrlEncode( SuccessUrl ) );


      if( CancelUrl != null && CancelUrl != "" )

            url.AppendFormat( "&cancel_return={0}", HttpUtility.UrlEncode( CancelUrl ) );


      return url.ToString();



The result from this method is then used in a Response.Redirect() that sends the customer off to PayPalís server. In Figure 2 you can see the transition. Notice that the redirection took you to a semi-custom page that has all of our order information and our company account info in it.


Figure 2: The first PayPal page prompts the user to login. Notice that all of our order information we passed is displayed on this form.


PayPal SandBox Note:

Iím testing transactions here in real time Ė actually I just used a $.50 amount and futzed the display above. You can also use the PayPal SandBox which is a simulated environment that runs against simulated accounts you can set up. There are a few minor differences in the way the forms display mainly due to differences in the way my test accounts are set up (which is why I use the real account). Why do you want to use the SandBox? Well, for testing you probably donít want to charge real accounts. More importantly,  tís difficult to test PayPal transactions if you donít have two separate accounts since PayPal wisely doesnít allow to send money to yourself <g>. You can set up a SandBox account from the PayPal Developer Network through the Merchant Tools page on your account.


At this point the user enters his password or if he doesnít have a PayPal account yet, he can create a new one at this point as well. Clicking the Continue button brings up the PayPal order confirmation page.


Figure 3: the PayPal order confirmation reviews the final payment status and letís your customer confirm the payment.


Once you click the Pay button a final confirmation page is displayed that confirms the actual payment having been made. This is the last page in the batch and clicking the continue button on the form shown in Figure 4 returns you back to the main application.


Figure 4: The final PayPal Confirmation page confirms the actual payment. The Continue link takes you back to the original site.


The return click uses the Success return URL that we provided as part of that long query string shown earlier. In this case this page returns to the following URL:




The West Wind Web Store then checks for the PayPal querystring value and based on it handles the processing of this order. Remember that when we originally sent off this request to PayPal we saved some information about the order form Ė the field values that were entered for the various form controls. We captured them to Session variables and now we simply retrieve them and reassign them to the appropriate controls to make the form appear very much like the form we left before we went off to PayPal.


The Page_Load() for the form contains a small block like this:



if (Request.QueryString["PayPal"] != null)



The HandlePayPalReturn() method then performs the page reconstruction by restoring the saved values from the session back into the form and then simulating a button click operation. In short, weíre restoring the state of the page to what it was prior to going off to PayPal and then clicking the button to make it behave just like a normal, local order process.


/// <summary>

/// Handles the return processing from a PayPal Request.

/// Looks at the PayPal Querystring variable

/// </summary>

private void HandlePayPalReturn()


      string Result = Request.QueryString["PayPal"];

      string Redir = (string) Session["PayPal_Redirected"];


      // *** Only do this if we are redirected!

      if (Redir != null && Redir == "True")




            // *** Set flag so we know not to go TO PayPal again

            this.PayPalReturnRequest = true;


            // *** Retrieve saved Page content

            this.txtNotes.Text = (string) Session["PayPal_Notes"];

            this.txtHeardFrom.Text = (string) Session["PayPal_HeardFrom"];

            this.txtToolUsed.Text = (string) Session["PayPal_ToolUsed"];


            if (Result == "Cancel")


// *** Redisplay the order page with an error message

                  this.ShowError("PayPal Payment Processing Failed");




                  // *** We returned successfully - simulate button click to save                           this.txtCCType.Text = "PP";      // Payment type to PayPal

                  if (!App.Configuration.CCPayPalAllowAutoConfirmation)

                        // Leave a visible note and cause not to Auto-Ship

this.txtOrderCode.Text = "PayPal"


                  // *** Simulate a button click






Notice the PayPal_Redirected Session variable which was set in HandlePayPalRedirection(). This flag allows to insure that this page was actually fired from a real PayPal request. Since weíre storing this value server side we can minimize spoofing attempts fired without actually initiating an order. Once we know that we are on a redirection we can remove the flag. We also set an internal flag Ė PayPalReturnRequest to true, so that any code that follows this method call knows that we processed this order. Specifically we want to make sure that we donít process credit cards in addition to PayPal, and more importantly that we redirect back to PayPal again.


The rest of the code deals with restoring the various text boxes with the saved values from the Session object. I would recommend if you have more than a few values that you use an object to store in the Session rather than single values, but since I only had 6 values including the flags I didnít bother. Using an object will make assignments and retrieval easier as it preserves type information of the fields plus it keeps the Session object more tidy.


Finally if all goes well we call the btnSubmit_Click() event handler method to essentially imitate a form submission. So now we know that weíve successfully processed a PayPal payment and we can act accordingly. Remember I say we Ďknowí Ė really weíre assuming at this point because we havenít verified that the PayPal indeed retrieved payment.


btnSubmit_Click() only has one hook to PayPal in it, which is the check for PayPal redirection mentioned earlier:


if (this.txtCCType.Text == "PP" && !this.PayPalReturnRequest)



This time around the this.PayPalReturnRequest flag will be true and weíll skip right past the redirection and proceed with the rest of the order processing. The business logic of this page is smart enough to know to not send PayPal orders to credit card processing, but if you needed to you could of course check either CCType or the PayPalReturnRequest flag to skip over any code you donít want fired when processing a PayPal return.


Otherwise the order processing is left completely unchanged and the end result of this operation is that the order gets processed and the customer will see an order confirmation page as shown in Figure 5.


Figure 5:
The order confirmation page displays the final order content on our e-Commerce site. Note that at this point you should not confirm any items to customers!


Remember this page should not disburse any products or access to services yet. If you recall the return URL from PayPal that is provided as part of the URL, so itís easy for anybody to see this URL and simply type it in even if the PayPal order did not go through.



You can also POST information to PayPal, which makes the request a little less prone to spoofing. But POSTing is significantly more complicated using ASP.NET since an ASP.NET page canít easily POST content to another page. Even then itís much more work to set up POST variables properly on a page. Rather, itís better to live with the possible tempering, but rely on the IPN confirmation or even manual checking on PayPal as the final evidence that the order went through.



Note that this process has hooked PayPal processing into the ASPX page with two simple if blocks Ė one at the end of the Page_Load() that handles a return request from PayPal:


if (Request.QueryString["PayPal"] != null &&

    this.txtCCType.SelectedValue == "")



and one in the button click that fires the redirection:


if (this.txtCCType.Text == "PP" && !this.PayPalReturnRequest)



All the rest of the code is completely isolated from the order processing logic. For completeness sake to understand the bigger picture, Iím providing the complete application specific Page_Load() and btnSubmit_Click() events of the OrderForm.aspx page here so you can get a feel for the order processing logic. This code makes extensive use of business objects so it should be easy to understand the flow:


private void Page_Load(object sender, System.EventArgs e)


      // *** Force the page NOT to cache



      // *** Remove the PayPal button if no PayPal Url is provided

      if (App.Configuration.CCPayPalUrl == "")



      this.Invoice = WebStoreFactory.GetbusInvoice();

      this.Invoice.TemporaryLineItems = true;


      InvPk = Session["InvPk"];

      if (InvPk == null)





      CustPk = Session["CustPk"];

      if (CustPk == null)


            this.ShowError("<hr>Error loading customer...");




      // *** create a new invoice object

      if (!Invoice.New(true,(int) CustPk))


            this.ShowError("<hr>Error loading shopping cart invoice...");




      DataRowContainers.wws_invoiceRow rowInv = Invoice.GetTypedDataRow(false);


      // *** Assign exiting Pks to this invoice

      rowInv.Pk = (int) InvPk;

      rowInv.Custpk  = (int) CustPk;


      if(!Invoice.LoadLineItems((int) InvPk))


            this.lblHtmlInvoice.Text = "<hr>Error loading shopping cart invoice...";

            this.txtCC.Text = "";




      // *** Retrieve the Shipping Info values

      Invoice.ShipInfo = (ShippingInfo) Session["ShippingInfo"];

      Invoice.ShipInfo.UseInvoiceFields = false;


      // *** Total out and display the invoice


      this.lblHtmlInvoice.Text = Invoice.HtmlLineItems(0,true);


      // *** Check for PayPal responses -

      // *** if we have no CC selection and PayPal QueryString we need to handle it

      if (Request.QueryString["PayPal"] != null && this.txtCCType.SelectedValue == "")




/// <summary>

/// Saves the invoice if all goes well!

/// </summary>

/// <param name="sender"></param>

/// <param name="e"></param>

private void btnSubmit_Click(object sender, System.EventArgs e)



      // *** Unbind the form and display any errors related to the binding


      if (this.bindingErrors != null)


            this.ShowError( this.bindingErrors.ToHtml());




      DataRowContainers.wws_invoiceRow rowInv = this.Invoice.GetTypedDataRow(false);


      rowInv.Ccexp = Request.Form["txtCCMonth"] + "/" + Request.Form["txtCCYear"];

      rowInv.Shipdisks = Invoice.ShipInfo.ShipToCustomer;


      if (Invoice.ShipInfo.ShippingMethod != "--")

            rowInv.Shipby = Invoice.ShipInfo.ShippingMethod;


      // *** Load up the Shipping Address captured previously

      object Temp = Session["ShippingAddressPk"];

      if (Temp != null)


            busShippingAddress ShipAddress = WebStoreFactory.GetbusShippingAddress();

            if ( ShipAddress.Load( (int) Temp ) )





      if (!this.Invoice.Validate())







      // *** Handle PayPal Processing seperately from ProcessCard() since it requires

      // *** passing off to another page on the PayPal Site.

      // *** This request will return to this page Cancel or Success querystring

      if (this.txtCCType.Text == "PP" && !this.PayPalReturnRequest)



      // *** Optional Credit Card Processing

      // *** the method decides whether CC processing actually takes place

      //if (App.Configuration.ccProces

      if ( !this.ProcessCreditCard() )


            string Error = Invoice.ErrorMessage;

            int lnAt = Error.IndexOf(";");

            if (lnAt > 0)

                  Error = Error.Substring(0,lnAt);


            // *** If we have a FAILED transaction pass it through unprocessed           

            // *** Otherwise echo error back.

            if (rowInv.Ccresult != "FAILED")


                  this.ShowError("Credit Card Processing failed: " + Error);





      // *** Save the invoice by copynig the Temporary LineItems to the real lineitems

      if (!this.Invoice.Save(true))






      // *** Clear out the Invoice Session PK so this invoice is 'history'   





      // *** Show the confirmation page




Instant Payment Notification

If you are processing orders only occasionally or you donít immediately fulfill orders online you might be done at this point. But remember that unless you check with PayPal or PayPal notifies you, you have no guarantee that PayPal actually processed the specified amount. More specifically your application doesnít know even if you receive an email from PayPal.


In order for your application to be securely notified of transactions on the server you need to implement Instant Payment Notification (IPN). In a nutshell, IPN provides a Web based call back mechanism for your application to independently receive confirmation from PayPal that a transaction was made to your account. PayPal POSTs back all the order information too, so you can verify that the order was actually confirmed for the amount that you originally asked for (another possible scam Ė exit the current order and come back in and transfer money for a different amount. Youíd actually get a confirmation but it wonít be for the right amount!)


Since PayPal calls this URL directly, the URL for this page is not immediately apparent and therefore more reliable. PayPal also includes information about the order back to you, so that you can double check that the important information Ė Invoice Number and OrderAmount most likely Ė matches what you thought you were having the user pay for.


IPN must be explicitly enabled on the PayPal site. The URL is configured like this: log on to your account, go to Profile | Selling Preferences | Instant Payment Notifications. Check the enable box and provide a URL on your site that you want IPN to post to. In the case above I want to have PayPal post back to my site to a special page called PayPalIPNConfirmation.aspx.


Hereís what a typical IPN POST looks like:

















As you can see thereís just about everything that was originally entered coming back to you plus some PayPal internal stuff. As I mentioned the things you probably want to look at is the invoice and payment_gross values which let you quickly validate that what you sent in is coming back to you.


This IPN POST back page should be a non-visual page ASPX page (or an HttpHandler) Ė you can remove all HTML code other than <%@PAGE %> directive. The code in this page receives what amounts to a POST back from PayPal that echoes back your order information. The implementation looks something like this in my Web Store:


public class PayPalIPNConfirmation : System.Web.UI.Page



      protected void Page_Load(object sender, System.EventArgs e)


            PayPalHelper PayPal = new PayPalHelper();


            busInvoice Invoice = WebStoreFactory.GetbusInvoice();


            // *** Reload our invoice and try to match it

            // *** a real invoice

            string InvoiceNo = Request.Form["invoice"];

            if (InvoiceNo != "")


                  if (!Invoice.LoadByInvNo(InvoiceNo))

                        // *** Nothing to do here





                  "Invalid PayPal Invoice IPN Invoice Number received:\r\n" +

Encoding.Default.GetString(Request.BinaryRead(Request.TotalBytes)) );




            // *** Send the response data back to PayPal for confirmation

            bool Result = PayPal.IPNPostDataToPayPal(App.Configuration.CCPayPalUrl,




            if (Result)


                  // *** Clear out the order PayPal order code

                  Invoice.GetTypedDataRow().Ordercode = "";


                  // *** Confirm the invoice if item is new order

                  if ( Invoice.CanAutoConfirm() )











                  catch {;}




Invoice.SendEmailConfirmation()). If it fails in anyway the error is logged into an application log..


IPNPostDataToPayPal() method. IPN works by having PayPal post back all the order information to you in POST format. The IPN protocol requires that your handler returns all the FORM variables PayPal posts to you by posting them back to PayPal. In other words we need to echo back all the POSTed form variables to PayPal, plus add a cmd POST value with a value _notify-validate to let PayPal know weíre returning an IPN signature.


Hereís the code in the PayPalHelper class that accomplishes this:


/// <summary>

/// Posts all form variables received back to PayPal. This method is used on

/// is used for Payment verification from the

/// </summary>

/// <returns>Empty string on success otherwise the full message from the server</returns>

public bool IPNPostDataToPayPal(string PayPalUrl,string PayPalEmail, decimal OrderAmount)


      HttpRequest Request = HttpContext.Current.Request;

      this.LastResponse = "";


      // *** Make sure our payment goes back to our own account

      string Email = Request.Form["receiver_email"];

      if (Email == null || Email.Trim().ToLower() != PayPalEmail.ToLower())


            this.LastResponse = "Invalid receiver email";

            return false;



      string Payment = Request.Form["payment_gross"];

      if (Payment == null)


            this.LastResponse = "Order Corrupt: Invalid payment amount.";

            return false;





            if (decimal.Parse(Payment,CultureInfo.InvariantCulture.NumberFormat) !=


            { {

                  this.LastResponse = "Order Corrupt: Invalid payment amount.";

                  return false;





            this.LastResponse = "Invalid order amount returned from PayPal.";

            return false;



      wwHttp Http = new wwHttp();



      foreach (string postKey in Request.Form)



      // *** Retrieve the HTTP result to a string

      this.LastResponse = Http.GetUrl(PayPalUrl);


      if (this.LastResponse ==  "VERIFIED" )

            return true;


      return false;




This code reads the incoming form vars and echos them back by looping through the Request.Form collection and simply writing each key back into the PostBuffer of the HTTP client. The code starts by performing a couple of validations against the incoming POST data. Specifically we check the PayPal account weíre receiving to and the order amount to make sure that there isnít any sort of spoofing going on (ie. the user decided to place a separate order possibly with a different order amount). If you need to do additional checks you can do that too by looking at the Form vars in the PayPalIPNConfirmation.aspx code.


The code then loops through all of the incoming Form vars and posts them right back to PayPal. Iím using a wrapper class around the .NET WebRequest class called wwHttp to simplify the posting process. wwHTTP automatically URL encodes form variables and returns a plain string result. You can find this class described and available for download in a previous article Retrieving HTTP content with .NET article. The class is also provided with the accompanying source code.


On success PayPal returns a simple value: VERIFIED. Now we finally have programmatically verified that the order is valid.


Debugging IPN

Testing the IPN verification is not trivial. Keep in mind that IPN works by having PayPal call back your application at a specific URL. This means if youíre testing and you want to use the debugger locally you will have to make sure that you work on a publicly accessible IP address that an external client (PayPal) can reach. If you sit behind a firewall, proxy or even an DHCP client itís not going to work. To make things worse I couldnít get IPN to work with the Sandbox Ė it only worked for me on the live site. The SandBox account had no IPN configuration options, which means you have to run live transactions to test/debug IPN. Make sure you place SMALL orders and donít accidentally order you most expensive items. The PayPal commission might kill Ďya <g>Ö


If you canít get a public ID to debug, you have to work the old fashioned way with messages dumped into Trace or Event logs or emailing yourself status messages from with in the IPN code. IPN is great, but plan on spending some time debugging this interface Ďhalf blindí if you donít have an open IP Address...

Configuration Management

Finally, if you integrate PayPal into your applications youíll probably want to provide some customization options for the processing, such as easy ability to turn PayPal processing on and off, set the PayPal Url (after all you might want to use the SandBox rather than the Ďliveí Url) or enable and disable IPN confirmations.


In the above code snippets that I have shown you can see the use of App.Configuration to hold configuration settings. These settings are stored in a Configuration Class that I described in a previous article. This class is handy as it maps properties/fields to entries in a configuration file including the ability to write out these settings. Figure 6 shows the Credit Card Processing Configuration Page.


Figure 6 Ė Itís a good idea to store changeable values in a configurable format in your application so you donít have to make code changes to switch behavior or accounts.


This is very important since itís very likely youíll want to change the above information while debugging for example. Changing the URL in this fashion is nice and easy and the Configuration class is one easy way to provide this and other configuration options through code.


Iíve spent a fair amount of time talking about my custom implementation in this article, because I think itís important to keep things in the perspective of a real application environment. I hope the code shown was not too specific and easily understandable in the context of the PayPal integration.

Iíve provided a really simple sample project that demonstrates the basic concepts Iíve described here along with the code to the helper classes mentioned, which you can run and play around with. Just remember to use small order amounts, eh? <g> The project contains a very different set of ASPX pages which are very basic and serve more as a skeleton that you can insert your application specific code into. You can download the sample code from:




Make sure you read the Readme.txt file that accompanies this project for setup and configuration instructions.


PayPal may not be the first choice for order processing in a custom e-Commerce application, but it does provide another payment option for you customers. In this article Iíve tried to pull together all the pieces you need to integrate PayPal into your own e-Commerce front ends in a way that is somewhat seamless. While none of this is new, I didnít find this information all in one place so I decided to write this up here for my own reference in the future as well as for you, eh?


If you have questions or comments about this or related topics, you can as usual post a message in the White Papers section of our Message Board.


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By Rick Strahl
Rick Strahl is president of West Wind Technologies on Maui, Hawaii. The company specializes in Web and distributed application development and tools with focus on .NET, Visual Studio and Visual FoxPro. Rick is author of West Wind Web Connection, a powerful and widely used Web application framework and West Wind HTML Help Builder and West Wind Web Store. He's also a C# MVP, a frequent speaker at international developer conferences and a frequent contributor to magazines and books. He is co-publisher of Code magazine. For more information please visit: http://www.west-wind.com/ or contact Rick at rstrahl@west-wind.com.

























































































































































































































































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