Paris is a large airport, so it’s not surprising that the postal services are decent. You may need to walk a while though.
The FAQ doesn’t help much.
According to the FAQ, drop boxes are available in each terminal. They do not seem to be located on any airport maps.
You can buy stamps at each Relay shop.
A post office is located in Terminal 2D by the car rental counters. This might be the one in the FAQ that you can use “in an emergency” (their words, not mine). I found this on the terminal maps.
Last updated 16 July 2017