IT:AD:PayPal:HowTo:Setup Post-Processing Page and Logic

The simplest: after the user pays, automatically (unless if purchasing by CC he'll always be presented with an transacted complete page that says “Return to Merchant” button. Hated by all…but it's a legal thingie that PaPal resues to change) redirected to a specified page on your website on which you can display some confirmation text.

A default can be setup under Menu/Profile/SellingPreferences/Website Payment Preferences

Under Profile/Selling Preferences/Website Payment Preferences
Turn Auto Return On and enter the Url
Save.

Can be overridden with a custom one embedded as Html Variable within the Buy button.

Will get posted back the following variables:

• tx - Transaction ID
• st - Payment status
• amt - Payment amount
• cc - Currency code

Eg:

http://www.yourdomain.com/Thanks.aspx?tx=[TransactionID]&st=[PaymentStatus]&amt=[PaymentAmount]&cc=[currenccode]

But the vars above are not enough to work with… You need to use PDT to perform a callback to the server, in order to get more info.

Under Profile/Selling Preferences/Website Payment Preferences

Turn Auto Return On and enter the Url (same as above).
Turn PaymentDataTransfer On, and you'll get a PDT Identity Token.
Save.

After following these steps, you should get a PDT Identity Token that is needed for querying PayPal.

If you don't copy-paste the token after clicking Save, know that you can always see it in your Website Payment Preferences:

You'll now get

http://www.yourdomain.com/Thanks.aspx?tx=[TransactionID].

which can then use to POST to:

https://www.paypal.com/cgi-bin/webscr?cmd=_notify-synch&tx=[TransactionID]&at=[PDTIdentityToken]

and you'll get back:

SUCCESS
first_name=Firstname
last_name=Lastname
payment_status=Completed
payer_email=firstname%40lastname.com
payment_gross=50.00
mc_currency=USD
custom=Custom+value+you+passed+with+your+HTML+form
etc.

In Code:

protected void Page_Load(object sender, EventArgs e)
{
    if (!Page.IsPostBack)
    {
        authToken = WebConfigurationManager.AppSettings["PDTToken"];

        //read in txn token from querystring
        txToken = Request.QueryString.Get("tx");


        query = string.Format("cmd=_notify-synch&tx={0}&at={1}", 
                              txToken, authToken);

        // Create the request back
        string url = WebConfigurationManager.AppSettings["PayPalSubmitUrl"];
        HttpWebRequest req = (HttpWebRequest)WebRequest.Create(url);

        // Set values for the request back
        req.Method = "POST";
        req.ContentType = "application/x-www-form-urlencoded";
        req.ContentLength = query.Length;

        // Write the request back IPN strings
        StreamWriter stOut = new StreamWriter(req.GetRequestStream(), 
                                 System.Text.Encoding.ASCII);
        stOut.Write(query);
        stOut.Close();

        // Do the request to PayPal and get the response
        StreamReader stIn = new StreamReader(req.GetResponse().GetResponseStream());
        strResponse = stIn.ReadToEnd();
        stIn.Close();

        // sanity check
        Label2.Text = strResponse;

        // If response was SUCCESS, parse response string and output details
        if (strResponse.StartsWith("SUCCESS"))
        {
            PDTHolder pdt = PDTHolder.Parse(strResponse);
            Label1.Text = 
                string.Format("Thank you {0} {1} [{2}] for your payment of {3} {4}!",
                pdt.PayerFirstName, pdt.PayerLastName, 
                pdt.PayerEmail, pdt.GrossTotal, pdt.Currency);
        }
        else
        {
            Label1.Text = "Oooops, something went wrong...";
        }
    }
}

One big shortcoming of the use of PDT above is that it if the user closes the browser (and remember that if the buyer is paying by CC, he'll be shown a “Return to Merchant” page on PayPal that he might close) your website will receive no notification of the sale, and not complete any post-processing logic.

That's why you are advised to combine the PDT with IPN for any serious integration with PayPal.