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