How can i send a letter to someone who doesnt have an address?

that probably didnt make any sense right. heres the deal, a couple weeks ago i went to mexico to visit some family. it turns out they live in a ranch in the outskirts of a desert. the houses there are very poor and they dont have streets, just dirt and rock roads. noone there has any numbers on their house because they dont even have doors. its a very poor part of mexico and they dont have much of anything. the only way they can go shopping is if they take the bus about three hours from where they live.

i know there must be an office of some sort or a bank or something where they can pick up a document that is sent to them. i just dont know where or how to do it. please help if u can. thanx