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