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