Here is the full list of all 25 POSTAL Redux achievements. It takes around 12-15 hours to unlock all of the achievements on Windows.
Perfect all Rampage levels.
Complete the Campaign having killed only hostiles.
Complete the Campaign with a total play time of 30:00 or less.
Complete every level in Co-op mode.
Complete all Rampage levels.
Watch the entire credit sequence.
Complete the Campaign.
Complete the Campaign having left no survivors.
Complete the Excess Postage campaign in Nightmare difficulty.
Kill all Ostriches in The Farm.
Wipe out the game protestors in Ghetto.
Kill 30 people with the Napalm Launcher.
Execute two of every hostile type.
Duck under 10 rockets.
Earn a kill with every weapon.
Reach the half way mark of the Campaign.
Blow up 50 barrels.
Commit suicide.
Finish a level with less than 10 health.
Kill four or more people with a single Rocket Launcher missile.
Kill three or more people with a single Mine explosive.
Firebomb the Marching Band.
Perform 13 executions.
When burning, set someone else on fire by running into them.
Make your first kill.