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.
Commit suicide.
Firebomb the band.
Make 100 kills.
Run for 5 minutes.
Enable cheats.
Start the second level.
Complete a level with less than 10 health.
Touch someone while burning.
Duck under a rocket.
Perform your first execution.
Take a rocket to the face.
Make your first kill.
Play the Mac or Linux version of the game.
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.
Watch all the credits.