Here is the full list of all 27 POSTAL achievements. It takes around 15-20 hours to unlock all of the achievements on Windows.
Enable cheats.
Fire 1,000,000 bullets.
Take 10,000 hits.
Hit 100,000 targets.
Make 10000 kills.
Play the Mac or Linux version of the game.
Kill a naked person.
Complete the game having only killed hostiles.
Complete the game using only the M16.
Watch all the credits.
Complete the game.
Complete the game having killed everything.
Make 1000 kills.
Use every weapon in the game.
Complete level 10.
Fight an ostrich.
Firebomb the band.
Duck under a rocket.
Make 100 kills.
Touch someone while burning.
Start the second level.
Complete a level with less than 10 health.
Commit suicide.
Run for 5 minutes.
Perform your first execution.
Make your first kill.
Take a rocket to the face.