POSTAL 2 had Steam trading card support added on July 22, 2013. There are 8 cards in the series, and you'll receive 4 cards at random for purchasing and playing the game.
The game was originally released as Postal 2 Complete. It was renamed with a later update. It's the sequel to POSTAL, which also has Steam trading cards. A third game in the series, POSTAL Redux also has trading cards.