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