According to the official information, you may want to drop off your postcards at a different airport or at a post office prior to arriving.
The interactive map states there are no drop boxes at the airport.
However, you can buy stamps post-security at a vending machine at the news stand in the center concourse. Thus while there’s no way to drop off a letter, you may be able to get the stamps you need as you’re exiting the airport.
Last updated 10 June 2017