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