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