It’s actually a nice airport Web site, but there’s no information about a mailbox or drop box. There’s also a little flight museum in the airport that looks interesting!
Robert mentioned “gray mailbox outside next to car rental door”.
Here’s the Google Maps link, set up so you can see the mailbox. It’s right outside the easternmost exit as you would walk to/from the car rental lot. That picture is from street view looking south.
Last updated 21 April 2022