Here is the full list of all 27 POSTAL achievements. It takes around 25-30 hours to unlock all of the achievements on Windows.
Hit 100,000 targets.
Make 10000 kills.
Play the Mac or Linux version of the game.
Enable cheats.
Complete the game having only killed hostiles.
Make 1000 kills.
Fire 1,000,000 bullets.
Firebomb the band.
Use every weapon in the game.
Watch all the credits.
Complete the game.
Complete the game having killed everything.
Complete the game using only the M16.
Take 10,000 hits.
Kill a naked person.
Touch someone while burning.
Complete level 10.
Fight an ostrich.
Duck under a rocket.
Complete a level with less than 10 health.
Make 100 kills.
Start the second level.
Run for 5 minutes.
Take a rocket to the face.
Perform your first execution.
Commit suicide.
Make your first kill.