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