Here is the full list of all 27 POSTAL achievements. It takes around 25-30 hours to unlock all of the achievements on Windows.
Enable cheats.
Hit 100,000 targets.
Fire 1,000,000 bullets.
Make 10000 kills.
Take 10,000 hits.
Play the Mac or Linux version of the game.
Kill a naked person.
Complete the game having only killed hostiles.
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.
Make 1000 kills.
Complete level 10.
Fight an ostrich.
Complete a level with less than 10 health.
Firebomb the band.
Start the second level.
Duck under a rocket.
Touch someone while burning.
Perform your first execution.
Take a rocket to the face.
Commit suicide.
Make your first kill.
Run for 5 minutes.
Make 100 kills.