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