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