The official site mentions a mailbox and a FedEx drop, “Between Southwest and American signs outside the main terminal”.
There are no mail facilities at the South terminal.
Robert emailed stating that there is a mail drop, “Blue Mailbox curbside upper level between the American Airlines and Southwest ticket counters”. (Robert updated in 2021 to clarify the location.)
A Google Maps search tells me where the FedEx box is, and a satellite image doesn’t appear with it. So note that it is outside around where the FedEx box is, and be mindful to not post items for USPS in the FedEx box.
Last Updated 22 November 2021