I recently ran into an issue where international customers could not checkout properly in Ubercart, a fully featured Drupal ecommerce solution. I was using Authorize.net as the credit card processer. To allow international customers to be verified with Authorize.net, do the following in your merchant account:
- Under account, click settings
- Under settings, click Address Verification Service
- Uncheck "G", "U", and "S"
According to this post on the Ubercart forums, these three values are the ones that prevent non-US addresses to be processed.