Here is the full list of all 26 POSTAL: Brain Damaged achievements. It takes around 10-12 hours to unlock all of the achievements on Windows.
Use an ammo box
Confuse at least 5 hostiles with piss
Use the steroids powerup
Kill Karen
Find a certain secret not on wednesday
Use the akimbo powerup
Use a health pipe
Freeze at least 5 hostiles with piss
Burn at least 5 hostiles with piss
Use portable armor
Pet a cat
Kill the big bad RONA without using the vaccine
Kill the big bad RONA
Kill Tesloborgelion
Find the third installment
Use the quick pillz powerup
Get 100% on any of the levels
Piss on Tesloborgelion boy parts
Find all enemy codex entries
Find all location codex entries
Find all weapon codex entries
Kill a certain group of hostiles in the forest
Kill a certain group of hostiles in the neighbourhood
Finish any level below the speedrun threshold
Find a certain secret on wednesday, my dude
Reflect 10 projectiles