This is the Geo-Post Office of Platanias. The Cache itself is hidden at the given Coordinates. This Letterbox is good for trading Postcards: Bring a Postcard with you, with the correct stamp on it and adressed to you. Trade it with another Postcard, which should be filled with your text and then sent from Platanias.
You can find real life letterboxes by the main street. Also there are stores that have postcards and stamps.
Anyone will be happy to get a postcard from a fellow cacher.