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