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