How to restrict customer entry of state to the two-letter postal code

We are on R12.1.3.  I want to restrict the entry of a customer's state value to the two-character postal code value, e.g., VA = Virginia.  I've tried using the setups for the United States in the Countries form, POSTAL_ADDR_DEF and POSTAL_ADDR_US, but neither enforce the two-character value be chosen.

Anyone know how to do this?

Thanks

Wayne