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