One of the most common reasons a credit card donation is declined is an "AVS mismatch".

An "AVS mismatch" occurs when the billing address provided during checkout does not match the billing address that the donor has on account with his or her credit card company or card-issuing bank.

This Address Verification Service is used to protect you against credit card fraud. 

To resolve this issue the donor should compare the billing address they enter with the one listed on their credit card statement. The address entered during the checkout process must match the credit card billing address and any difference between the two addresses can result in an AVS mismatch and thus a declined transaction. 

Note: When a donor receives an AVS mismatch error, the card issuing bank or credit card company typically puts a hold on the card to cover the donation amount. The donor's card was not charged. Their credit card company or card-issuing bank will release the funds, typically within 2-3 business days. If the donor has questions regarding when the funds will be posted back to the account they should follow up with their bank.