If you have regular mail that is pre-stamped and can be dropped in a standard USPS drop box, there are a few: in front of the commuter terminal, at Terminal 1 in front of Southwest & United Airlines, and at Terminal 2 near each of the 2 red bus stops.
The port for shipping is nearby. There are courier services of course
Nope
There are none in the airport but several located around the airport
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