The Passenger Services page has detailed information on the mail drop and stamps:
The U.S. mailbox is located outside along the curb, just north of the main entrance to the terminal building. Stamps can be purchased at the Adventure News store in the terminal lobby or through any of the Wells Fargo ATMs.
Note that buying stamps from an ATM may incur an additional surcharge, and are generally limited to books of twenty (20) USPS First Class Forever stamps.
The first-floor terminal map (PDF located under “Helpful Maps” on their site) shows where the drop box is; there’s also a FedEx drop box next to it.
Robert gives a better description on the drop box:
Located curbside baggage claim, standard blue mailbox.
Google Maps shows exactly where it is curbside, just outside of the main entrance.
Last updated 19 August 2018