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