post office

Is there such a thing as PO Box rentals here like a Mailbox, etc. or like UPS stores have in the USA? I am traveling around and don't have a stable address. Does something like this exist? What do most of you do that move around and need to get stuff mailed here?

I met up with an American that has been living in Buenos Aires the past 15 months. It was an acquaintance of a friend. He did not speak Spanish at all but he claims that he ordered something from the USA and his mom sent it via the post office and it was just a book and worth almost nothing...