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