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