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.
Make 1000 kills.
Fight an ostrich.
Complete a level with less than 10 health.
Touch someone while burning.
Firebomb the band.
Take a rocket to the face.
Duck under a rocket.
Make 100 kills.
Run for 5 minutes.
Perform your first execution.
Commit suicide.
Start the second level.
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.