Customer's zip code (or postal code if outside of the USA).
This field is used as part of the Address Verification Service (AVS). If the customer resides within the United States, this field should contain the five or nine digit zip code as it appears on the customer's monthly statement. If the customer's billing address is outside of the United States, this field should contain the customer's postal code.
This field is optional and has a maximum length of 9 characters.