Their information page has some interesting notes:
After 9-11 letter drops became restricted in the terminal. Postage stamps can also be purchased at Cherry Street Travel Mart. There is a post office located next to the Cell Phone lot.
It sounds like you can buy stamps at the airport but not send something from the airport. But it also implies there may be places to send letters within the terminal.
There’s also a FedEx drop box across the hall from the restrooms at the security checkpoint; it’s possible a USPS drop is located in the same area.
The post office isn’t terribly far away from the terminal, but exercise caution if you choose to walk to it. It’s about a ten minute walk for most people.
I wasn’t able to find drop box at terminal curbsides from Google Maps.
Last updated 19 August 2018