At the top of the post office is a box of carrier pigeons that you'll have to re-arrange. Before you can do this you must first get the Magnetic Field Machine item from the Mayor's Office.
Head to the Mayor's Office across the hall from the Post Office. Once inside use the bell to open up the Mayor's Desk. If it's still the hidden drawer than use the Divining Rod on the desk to turn it back into the bed. Now check the cushions to find the Magnetic Field Machine, take it.
Return to where the pigeons are and place the Magnetic Field Machine in the cupboard next to the pigeons. Interact with it to start the puzzle.
You'll now be met with a grid with buttons. You'll have to hit these buttons in a certain order to complete the puzzle. Here'a an image with the buttons marked A through D and the order you'll have to press them in:
Follow this order exactly:
- Press C 1 time.
- Press B 2 times.
- Press A 1 time.
- Press B 2 times.
- Press C 1 time.
- Press B 2 times.
- Press D 1 time.
After that press
to exit the puzzle and your achievement will pop.