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