The article pays special attention to the process of automatic payment verification using IPN (Instant Payment Notification). The article is based on the experience of KB_Soft Group and the official PayPal documentation.
Types of PayPal Payments
PayPal supports several types of payments:
Payments for goods in the PayPal cart. PayPal is responsible for all operations supporting the cart in this case. Unfortunately, this option does not provide the maximum flexibility that is required to implement some projects, so the article does not consider this option.
"One click" shopping. Goods are not put into the cart in this case. This method also is used to pay for goods in a cart that was filled without PayPal. That's why this option provides maximum flexibility and full control of the cart.
Recurring billing or subscription. PayPal provides a subscription capability; that means that a definite sum will be periodically transferred from the user's account to the seller's account. The user can unsubscribe anytime. The seller can specify the subscription's period and cost. He also can organize a trial period to let the user assess the quality of services he provides. The trial period can be either paid or free.
For the reasons described above, the article will consider the second option. The subscription will not be described to make the example simple. To interact with the PayPal system, KB_Soft Group uses a UserControl that is a product internally developed for these purposes. The given example of the code that works with PayPal uses a special HTML form for the request to make clear the explanation of interaction with the system. PayPal also provides its own control as a dynamic library, but, unfortunately, this control functions correctly only with American locales (en-us). Besides, it does not provide full functionality to work with PayPal. Neither does it provide the flexibility required to work on some projects.
The Payment Process
The payment process is very simple. A POST form is created with a set of hidden fields that contain information about an item (identifier, name, and cost) and a button to send the form. It should be noted that all prices should be expressed with two digits after the decimal point. If an item costs $10, its price should be expressed as "10.00". When the form is sent, the buyer goes to the paypal.com site and finishes the payment process. When a real PayPal account is used, the form should be sent to https://www.paypal.com/cgi-bin/webscr. The developed example also allows you to work with the PayPal Sandbox. Use the "UseSandboxbo" parameter of web.config and the form will be sent to https://www.sandbox.paypal.com/cgi-bin/webscr.
