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