To unlock this achievement, you need to do the following:
- Deliver post to all 20 postboxes across the map.
- Deliver the three overdue packages (available after you've delivered a certain number of postboxes, I think 10).
You then have to return to the post office and speak to the Parrot, who will give you permission to claim the Postmaster badge. Climb up the ladder inside the building and press
to collect. Achievement unlocked!
Video guide to all postboxes here (all credit to xWinG):
Couple of points to note on the overdue packages:
- When delivering Sin's package, he doesn't like to be disturbed, so you need to shoot yourself around to the left side of the house from the island directly below his house, then drop the package through the floor above him.
- To deliver The package on the right requires you to have the bumblebee ball paint, which is made by combining the Sweet Sprinkle and Creepy Sprinkle.