A couple of our merchants helped each other find some useful info, so we decided to add this page from their info...
How do I know what you accept (and what you expect)? In my test cases, I put "USA" for a country, and it didn't go through. I didn't realize it was using a two-character "US" code.
(from @kfrancis) The list of codes is readily available online (ISO 3166-1 Alpha 2), and in the .NET wrapper Chargify.NET I provide that data built in as either just a dataset or a custom dropdown control that auto-populates the codes.