One of the most common reasons international credit cards have trouble in Populi is something called an "AVS mismatch". An AVS mismatch produces an error stating that, "The address provided does not match the billing address". AVS stands for Address Verification System, and it is a protocol used within the United States that helps Authorize.net verify the address of the person using the credit card. However, it is not widely used outside of the United States and thus causes problems with international credit cards.
The solution is to go to your Authorize.net user control panel and disable automatic rejection of transactions that fail AVS.
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