I just wrote a C#-backed ASP.NET PayPal backend. I'm not going to post it because it was so trivial that I just made it totally custom to my needs. Wow, C# rocks, ASP.NET rocks, and this is an indication of why Web Services rock (even though the PayPal back-end is HTTP POST instead of a decent SOAP call). Less than an hour between "Is this possible?" and a functioning mini-payment scheme.