Here is the full list of all 27 POSTAL achievements. It takes around 25-30 hours to unlock all of the achievements on Windows.
Kill a naked person.
Enable cheats.
Complete the game.
Watch all the credits.
Make 1000 kills.
Use every weapon in the game.
Complete level 10.
Fight an ostrich.
Firebomb the band.
Run for 5 minutes.
Duck under a rocket.
Commit suicide.
Complete a level with less than 10 health.
Make 100 kills.
Start the second level.
Touch someone while burning.
Take a rocket to the face.
Make your first kill.
Perform your first execution.
Make 10000 kills.
Fire 1,000,000 bullets.
Hit 100,000 targets.
Take 10,000 hits.
Complete the game having killed everything.
Complete the game having only killed hostiles.
Complete the game using only the M16.
Play the Mac or Linux version of the game.