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