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