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