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