I was playing with ComponentOne's Paypal ASP.NET control this night, and it's quite cool! It lets you easily add AddtoCart and Checkout functionality to a website.

BTW - the control is part of the *free* [sorry, had to] Visual Basic Resource Kit, and even comes with C# samples for those who may be worried about something called a “Visual Basic” Resource Kit. The controls work everywhere. Well worth the download.

Epilogue: Sadly, my unnamed hoster prevents me from using the control (they run under low trust -- it's stopped me running fun stuff in the past), so I had to 'liberate' their Javascript, and do it myself. Ah, well, job done, client (wife) better be happy....