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