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