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