How AVS Works:
When a customer submits a credit card transaction in the shopping cart, the payment gateway (not the shopping cart) compares the numbers in both the address and zip code submitted on the store's checkout page against those on file with the credit card company. The payment gateway will make a decision on whether to approve or decline the credit card transaction based on how closely the submitted values match those on file.

Most payment gateways provide options to control how strictly their AVS rules are enforced.