POSTNET is the barcode that represents the U.S. zip code. Its character set is '0' to '9'. Though not part of the character set, the dash ("-") can be used (optional), when working with a 9-digit ZIP code, to separate the first 5 and the last 4 digits. A one digit check character is used.

As of this page being written, the standard requires an 11 digit data: ZIP + 4 + 2 digits with the last two being the last two digits from the steet number. However, this product does not enforce data validity based on the data length, leaving it to the discretion of the application programmer.