The Playa del Coco Correos or Post Office is on the main road on the right side next to the anchor.
There is no postal delivery system that brings packages to your home. Instead you go pick up your mail at the Correos.
To get mail you address it to (paper was from the postman at the Correos):
Correos Playa deal Coco
Guanacaste Carrillo Sardinal