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