Thanks to Robert for pointing out where the mailbox is!
There’s nothing written on the page but it’s on the terminal map (PDF). It’s on the second floor behind the ticket counters, but before you enter the “Civic Plaza” atrium that connects the two terminals pre-security.
Robert emailed and submitted this information that may be more useful:
Second floor across from American Airlines ticket counter near information desk
Last Updated 18 August 2018