Rick Hunter, voice of the Postal Dude in the first two games, wasn't present to voice the Postal Dude. In a recent (2017) interview, he claimed it was due to depression from a breakup with his girlfriend at the time.
The game was released early mid-development by Akella. As such, many promised features were never implemented (such as multiplayer), and the game tends to crash in many areas, especially in places with lots of gunfire and NPCs. Many players report a common crash with the final boss. Akella has yet to patch it.
Running with Scissors retconned the game in their 2016 Postal 2 (2003) DLC Postal 2: Paradise Lost (2015), claiming that it took place within a dream the Postal Dude had during a coma.