I was looking at the Amazon Flexible Payment System service API and it's a whopping big set of docs. Unfortunately, I was sorely disappointed to see the operations in the HTTP based API (called a REST Request) seem to all be GET requests with an Action= query term. Ugh.
The documentation doesn't even mention what HTTP method to use, what content-type to submit or expect as a return, etc. I'm pretty sure that they simply don't know the difference. Sad but true. At least there are docs.
I suppose they didn't have the time to make it simple.