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