PostNet is the older USPS barcode for the ZIP code: a row of tall and short bars along the bottom of an envelope. Since 2013 the USPS has asked for the Intelligent Mail barcode instead for its automation prices, but PostNet is still read.

ZIP code, 5 digits

Delivery point, 11 digits

What it holds
Characters	Digits only, with no hyphen.
Length	5 digits (the ZIP code), 9 (ZIP+4) or 11 (the delivery point code).
Check digit	Always there, worked out by Labeljoy. It's in the bars, not in the readable text.
Readable text	The digits, under or over the bars.

The rest of the settings - text position, quiet zone, color - are in the Barcode section.

What goes wrong
Invalid input data length.	The value isn't 5, 9 or 11 digits long.
Numeric only data expected.	The value has something other than digits - a hyphen in a ZIP+4, for instance.