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