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As with POSTNET, the check digit is calculated by summing the other characters and calculating the single digit which, when added to the sum, makes the total divisible by 10.
The check digit can also be expressed as the sum, including the check digit, modulo ten equals zero. For example, for my own ZIP code, 14210, the sum (and coincidentally, the sum modulo 10) is 8, so the check digit is 2. For the ZIP code where I used to live in Cheektowaga, 14225-2105, the Postnet check digit would be 8.