It would be nice if we could print POSTNET barcodes.  Here is the specification
for the POSTNET barcodes that's in the USPS online documentation:
1.1
POSTNET (Postal Numeric Encoding Technique) is the USPS-developed barcode
method to encode ZIP Code information on mail that can be read for sorting by
automated machines. A POSTNET barcode can represent a 5-digit ZIP Code (32
bars), a 9-digit ZIP+4 code (52 bars), or an 11-digit delivery point code (62
bars).  The information content of the barcode is based on the combination of
tall (full) bars and short (half) bars. A tall bar represents 1, and a short
bar represents 0. When separated into groups of five, these bars sequentially
represent each of the digits of the ZIP Code (or ZIP+4 code or delivery point
code) for the delivery address, plus an additional digit designated as the
correction digit. The correction digit is derived from adding the numbers in the
ZIP Code (or ZIP+4 or delivery point code) and determining which single-digit
number must be added to that sum to make the total a multiple of 10. The first
and last bars of the barcode are frame bars and must always be full bars.
5-Digit Barcode
1.2
A 5-digit barcode is a single field of 32 bars consisting of a frame bar, a
series of 25 bars that represent the correct 5-digit ZIP Code for the address on
the piece, 5 bars that represent the correction digit, and a final frame bar.
ZIP+4 Barcode 
1.3
A ZIP+4 barcode is a single field of 52 bars consisting of a frame bar, a series
of 45 bars that represent the correct ZIP+4 code for the address on the piece, 5
bars that represent the correction digit, and a final frame bar.