If you have a merchant account with a US bank, it is likely that your Payment Gateway has been set up with AVS - Address Verification Software - for your security. This system will try to verify that the cardholder name and address match before processing the transaction. If the address is outside the US, the transaction may be automatically rejected as the address cannot be matched by US banks.

For Authorize.net:
Please read the instructions provided by Authorize.net in their online help regarding AVS settings on your Authorize.net account. 

If an international card has been declined by Authorize.net, you should try removing the settings for G, S and U. If that doesn’t work, remove B and/or N.

For all other US-based Payment Gateways:
Please refer to their help documentation when logging into your gateway account.