You may have already heard that Mastercard is rolling out cards that start with a “2” instead of the traditional “5.” You’ll want to check your version of Zen Cart to be sure you can handle this. Versions 1.5.5b and above have built in logic for the native payment methods (such as Authorize.net); if you are running a lower version, you will need to merge in the changes from the latest copy of includes/classes/cc_validation.php. If you are using an extension to manage credit card payments, be sure to test with Mastercard 2223000048400011. Then you can have confidence that your cart properly processes “2” series Mastercard cards.