Thanks to Robert for submitting this link to me!
There are mail services at and nearby the airport, and their Airport Services page does a good job of describing them:
A FedEx, UPS and US Postal Service mail box are conveniently located at the bottom of the escalator in the center observatory and the mail is collected daily. In addition, there is a full-service US Post Office conveniently located five minutes from the airport entrance at 1485 Howard Road. Its lobby area is open 24 hours every day.
Robert noted it was a blue mailbox, with pickup times Monday-Friday at 3:30pm. (No weekend or holiday pickups.)
Last updated 22 November 2021