Here is the full list of all 27 POSTAL achievements. It takes around 15-20 hours to unlock all of the achievements on Windows.
Make your first kill.
Start the second level.
Duck under a rocket.
Run for 5 minutes.
Perform your first execution.
Make 100 kills.
Watch all the credits.
Take a rocket to the face.
Enable cheats.
Commit suicide.
Touch someone while burning.
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.
Firebomb the band.
Kill a naked person.