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