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