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.
Use every weapon in the game.
Make 1000 kills.
Complete level 10.
Fight an ostrich.
Commit suicide.
Complete a level with less than 10 health.
Firebomb the band.
Make 100 kills.
Duck under a rocket.
Run for 5 minutes.
Touch someone while burning.
Start the second level.
Perform your first execution.
Take a rocket to the face.
Make your first kill.
Make 10000 kills.
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.
Watch all the credits.
Play the Mac or Linux version of the game.
Kill a naked person.