I am publishing this post as a hobbyist, because I find barcodes cool (as demonstrated by a prior open source project using 2D barcodes that I worked on). Do not attempt to use this information for any illegitimate purpose. Counterfeiting postage is a federal crime. 18 U.S.C. § 501.
This post ties together some of my favourite things: 2D barcodes, high speed automation, printers, cryptographic signatures (!), postal mail and postage, fraud prevention, and even a little bit about patents!