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