Are there international restrictions for "Check Out as a Guest"?

So far we're very pleased with this Pay Per View plugin. As far as I can tell it's working flawlessly.

However, we just received a message from a reader in Brazil that he can't check out as a guest. According to him, when he clicks the "Check Out as a Guest" button, it takes it back to our site. I've tried it and it works fine for me, but I'm in the US.

Here's one of our book pages - in case you want to check it out. Any insight or ideas I might check would be appreciated.

Kind regards,