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