If you are using anything besides 1.5.5e or 1.5.5f, you may need to grab that file for MasterCard. Mastercard is starting to use card numbers that start with a two instead of a five. The 1.5.5e file that validates Master Card numbers correctly can simply be used instead for 1.5.4. That file is includes/classes/cc_validation.php. If you are using an earlier version of Zen Cart (1.5.3 and below), you may need to do some file integration but I recommend upgrading to at least 1.5.4 instead.

I'm still not installing 1.5.5 as my default choice but as changes to 1.5.4 increase, that may change. They are still putting out corrections and updates to that version and I really want it to be settled before I use it consistently. The changes from 1.5.4 are not enough to warrant upgrading to 1.5.5 yet.