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