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