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