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