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