I'm sorry, but this seems very much like a hack to me as well.

I acknowledge this issue is a bit of a thorn in the side of ASP.NET web forms. But I've come up with two approaches that seems less "hacky" to me:

http://www.softcircuits.com/Blog/post/2009/01/22/Implementing-Non-ASPNET-Posts-in-ASPNET.aspx

and

http://www.softcircuits.com/Blog/post/2010/02/14/Quick-and-Dirty-Buy-Now-Buttons-in-ASPNET.aspx

The one that makes most sense depends on your requirements. Note that these are simply Buy-Now buttons I'm discussing. For anything more involved, you should either delve fully into the PayPal ecommerce API or write your own.