Here is the full list of all 27 POSTAL achievements. It takes around 15-20 hours to unlock all of the achievements on Windows.
Complete level 10.
Fight an ostrich.
Make 1000 kills.
Complete a level with less than 10 health.
Firebomb the band.
Run for 5 minutes.
Make 100 kills.
Touch someone while burning.
Perform your first execution.
Start the second level.
Commit suicide.
Duck under a rocket.
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.
Use every weapon in the game.
Watch all the credits.
Play the Mac or Linux version of the game.
Kill a naked person.
Enable cheats.