• get a single player map from the beta version of postal 2 for example "EastMall.♥♥♥" (if you want to play in the mall)
  
• add 'MPDGT-Coop' to the start of the map name, for example "MPDGT-CoopEastMall.♥♥♥"
  
• the map must also be placed in the share the pain 'maps' folder which should be here C:\Program Files (x86)\Steam\SteamApps\common\POSTAL2Complete\ShareThePain\Maps"

you dont have to add the "Coop" part, i just perfer to as it keeps all the co-op maps together
also you should have no spaces, only numbers and letters in the map name after "MPDGT-"

as long as everyone sticks to this, they will count as the same multiplayer map, but remember the map has to be from the 1409/beta version of postal 2

NOTES:

• you have to set up the server, or join a server on the multiplayer/share the pain mode
  
  
• i recomend playing on death match/free-for-all, so your friends can still kill eachother for fun
  
  
• you should turn scoring and bots off, bots get annoying and scoring sucks as every time you kill a npc it counts as a player kill, so if you kill like 25 npcs the game will end
  
  
• if a player commits suicide, or is killed; he or she will respawn like in a multiplayer game
  
  
• also all ammo/weapons respawn after 15 to 30 seconds, however i dont think NPCs respawn
  
  
• you should probably have matches timed so co-op maps rotate before all NPCs are killed (alternativley the host can change the maps in the admin settings in the pause/escape menu)

Link: http://www.mediafire.com/download/bjimykzraab5l8k/Postal+2+STP+Co-op+Map+Pack.7z
UPDATED WORKSHOP LINK (contains even more maps): http://psteamcommunity.yuanyoumao.com/sharedfiles/filedetails/?id=458981749

Notes

• if you are not hosting you will get weird textures on NPCs as seen in my video below
  
• Just Updated the download and added the map with the dairy its "suburbs1"
  
• The Mall is only the east half, but has the weapon shop c:
  
• i think all the maps are set to friday, coz they seem to have alot more weapons in them then they would have in other days
  
• the police station is a difficult map, but there is a LOT of wepons in the evidence room
  
• im posting here coz i have no idea how to put them on the workshop

Sorry for my bad english lol, its not my best subject (ive practically failed it)
also heres a youtube vid of me and my friend playing co-op
feel free to subscribe to my channel: https://www.youtube.com/user/CozaRawwwrrzBjtch