Hi Mike,
There are multiple ways we validate the card number, but the length is not one of them because it is not formally specified by the card companies. Every Visa card you ever see may be 16 digits, but there is nothing in their standards that says a 15 digit Visa is absolutely invalid.
We primarily validate the following:
1. We perform a mod-10 mathematical check on the card number to confirm that it at least has the possibility of being valid.
2. We confirm that the bin number (first 6 digits of the card) is valid and assigned to one of our supported card types. It is the BIN number that tells us the card type.
Things that are validated on production accounts are the card number (making sure its valid), expiration date (needs to be a future date), the card code and address—both will be verified if your settings are set to check this (Merchant Interface-Account tab-Security Settings- you will see AVS and Card Code Settings). You do not need to match the length of the credit card to the card type because you don’t need to pass the card type—it is automatically determined based on the credit card number entered.
Thanks!
-Joy