Here is the full list of all 27 POSTAL achievements. It takes around 15-20 hours to unlock all of the achievements on Windows.
Enable cheats.
Fire 1,000,000 bullets.
Take 10,000 hits.
Make 10000 kills.
Hit 100,000 targets.
Play the Mac or Linux version of the game.
Firebomb the band.
Make 1000 kills.
Kill a naked person.
Duck under a rocket.
Complete the game having only killed hostiles.
Watch all the credits.
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.
Complete a level with less than 10 health.
Make 100 kills.
Touch someone while burning.
Run for 5 minutes.
Commit suicide.
Perform your first execution.
Start the second level.
Take a rocket to the face.
Make your first kill.