I just completed a website where the client uses PayPal to handle the payment transaction. I have used PayPal quite a bit in the past, but I have always had problems using IPN (Instant Payment Notification). Because of the problems, I usually opted not to use IPN, but I was determined to use it for this project. The primary problem with testing IPN is, because the transaction occurs in the background, it is difficult to figure out where the point of failure is. But that is all behind me now.