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