To gain access to the command console, required to enter cheats, follow these instructions:
Create a shortcut to Postal 3 from the directory of the game, found where you installed it. With that done, right click and go to properties. Add ' -console' to the end of the shortcut location, without the quotes.
Your file should then be C:Program Files (x86)SteamSteamAppscommonpostal iiip3.exe -console
In Steam, you can also enter your library, select properties and then change the launch options location to have '-console' on the end.
With that done, one way or the other, you can press the tilda key (~) in game to access the console. Then type sv_cheats 1 to turn on the codes.