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