With the franchise comes merch and what-not. RWS has an official store (that is not really good to be viewed by those under 17). In the store they sell character dolls, which are their most famously known sold item, games (SIGNED!), cameos, shirts, stickers, and more!
You can also buy the games on PC straight from steam, including mods and game-packs. (and a little more)

The Book(s)
The Movie

Games On Steam
Postal
OST
Package

Postal 2
Paradise Lost
Corkscrew RuLes!
Full Collection
OST
Pinball Table Pack

Postal III
OST

Postal Redux
OST
Pinball Table Pack

Postal 4: No Regerts
OST

Postal Brain Damaged
Art Book
OST
Connoisseur Ver.

Collabrative/non timeline related
Postal Babes
POOSTALL Royale
Holy Postal!
DUDE on DUDE
DUDE on DUDE vol. 2