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