Then why is my barcode so short?

You can see this quite easily.
Print this on a report form using font sc postnet.

m.cstring is all the digits in your zip
m.nsum is the sum of all those digits.

m.cString = '|' + m.cString + CHR(ASC('0') + (10 - (m.nSum % 10)) % 10) + '|'

Compare the barcode you printed to the barcode on a piece of mail you received from the post office.
Is your barcode the same or is it shorter?

Deb