Normally, you would block certain countries for a setup like this. A standard country blocker type plugin for your WordPress site should work for this. Something like this maybe:
Also, you can put a message on your site about which countries you support. You can add it to your terms and conditions too.
Try it and see if you get too many transactions from other countries then let us know. We should be able to create some kind of addon to cater for this (if it becomes a big issue).