Here is the full list of all 27 POSTAL achievements. It takes around 25-30 hours to unlock all of the achievements on Windows.
Run for 5 minutes.
Watch all the credits.
Duck under a rocket.
Touch someone while burning.
Take a rocket to the face.
Commit suicide.
Play the Mac or Linux version of the game.
Make your first kill.
Start the second level.
Perform your first execution.
Make 100 kills.
Make 1000 kills.
Make 10000 kills.
Complete level 10.
Complete the game.
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.
Complete a level with less than 10 health.
Fight an ostrich.
Firebomb the band.
Kill a naked person.
Enable cheats.