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.
Execute two of every hostile type.
Complete every level in Co-op mode.
Blow up 50 barrels.
Kill three or more people with a single Mine explosive.
Wipe out the game protestors in Ghetto.
Kill all Ostriches in The Farm.
Finish a level with less than 10 health.
Perform 13 executions.
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.
Reach the half way mark of the Campaign.
Complete the Campaign.
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.
Complete all Rampage levels.
Commit suicide.
Watch the entire credit sequence.
Firebomb the Marching Band.
Earn a kill with every weapon.
Duck under 10 rockets.
Kill 30 people with the Napalm Launcher.
Perfect all Rampage levels.
Complete the Excess Postage campaign in Nightmare difficulty.