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