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