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.
Fire 1,000,000 bullets.
Hit 100,000 targets.
Make 10000 kills.
Complete the game having only killed hostiles.
Complete the game using only the M16.
Watch all the credits.
Complete the game having killed everything.
Kill a naked person.
Take 10,000 hits.
Complete the game.
Use every weapon in the game.
Enable cheats.
Duck under a rocket.
Make 1000 kills.
Complete level 10.
Fight an ostrich.
Complete a level with less than 10 health.
Run for 5 minutes.
Make 100 kills.
Firebomb the band.
Touch someone while burning.
Start the second level.
Commit suicide.
Take a rocket to the face.
Perform your first execution.
Make your first kill.