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