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