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