For those of you who don’t get the “Conch Republic” reference, here is the Wikipedia article with a picture of the sign over the airport.
The airport services page does a good job of noting where the mail drop is:
Outgoing mail may be dropped off at the USPS mailbox located outside the arrivals level on the lower roadway, at the midpoint of the terminal building.
The terminal map also shows the location, but it’s more difficult to see if it is inside or outside of the building. (The yellow icon coloring isn’t great from a readability standpoint either.)
Robert wrote in and mentioned that the drop box is inside, next to the restroom in the baggage claim area.
Unfortunately Google Maps doesn’t have any terminal pictures.
Last updated 19 August 2018