The postnet number is the 9 digit zip code, plus the code point (usually the last two digits of the house number), plus a parity digit (ten minus the sum modulus ten).

http://en.wikipedia.org/wiki/POSTNET

Here's a perl script to do it to zip+4 97212-2744 code point 03, parity 9.

[code]
perl -ne '@h=("┃┃╻╻╻","╻╻╻┃┃","╻╻┃╻┃","╻╻┃┃╻","╻┃╻╻┃","╻┃╻┃╻","╻┃┃╻╻","┃╻╻╻┃","┃╻╻┃╻","┃╻┃╻╻");s/./$h[$&]/ge;chop;print "┃$_┃\n"' <<< '972122744039'
┃┃╻┃╻╻┃╻╻╻┃╻╻┃╻┃╻╻╻┃┃╻╻┃╻┃╻╻┃╻┃┃╻╻╻┃╻┃╻╻┃╻┃╻╻┃┃┃╻╻╻╻╻┃┃╻┃╻┃╻╻┃
[/code]