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