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