After knowing whats a billing postal code, it’s important to note that i ts primary function is to ensure the accuracy of transactions and guard against fraud by verifying the billing address given matches the one linked to the account or payment method.. Asking what is the billing zip code is a common security measure in online shopping and other payment processes to confirm the location of A credit card postal code is an additional form of security used to verify it is being used by the owner of the card or an authorized user. It is connected to the five digit zip code for the billing address of the cardholder. QX4Q2.