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