Here is the full list of all 27 POSTAL achievements. It takes around 15-20 hours to unlock all of the achievements on Windows.
Fire 1,000,000 bullets.
Kill a naked person.
Take 10,000 hits.
Complete the game having only killed hostiles.
Hit 100,000 targets.
Firebomb the band.
Play the Mac or Linux version of the game.
Enable cheats.
Make 10000 kills.
Use every weapon in the game.
Make 1000 kills.
Complete the game.
Complete the game having killed everything.
Complete the game using only the M16.
Watch all the credits.
Complete a level with less than 10 health.
Complete level 10.
Fight an ostrich.
Touch someone while burning.
Duck under a rocket.
Make 100 kills.
Perform your first execution.
Run for 5 minutes.
Start the second level.
Commit suicide.
Take a rocket to the face.
Make your first kill.