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