Here is the full list of all 27 POSTAL achievements. It takes around 25-30 hours to unlock all of the achievements on Windows.
Enable cheats.
Fire 1,000,000 bullets.
Play the Mac or Linux version of the game.
Complete the game having only killed hostiles.
Take 10,000 hits.
Hit 100,000 targets.
Make 10000 kills.
Complete the game having killed everything.
Kill a naked person.
Watch all the credits.
Complete the game.
Complete the game using only the M16.
Make 1000 kills.
Use every weapon in the game.
Complete a level with less than 10 health.
Duck under a rocket.
Complete level 10.
Fight an ostrich.
Commit suicide.
Take a rocket to the face.
Make 100 kills.
Firebomb the band.
Touch someone while burning.
Run for 5 minutes.
Start the second level.
Perform your first execution.
Make your first kill.