When I was living in BA last year I had a few care packages sent to me by my family. They mostly only contained hot sauce, tea, chocolate, etc. Every time I would get a notice at my house saying that my package was at the Retiro mail center so I always had to go down there to retrieve it.
It's a pretty annoying process. You have to first wait a while to receive a number. Then you go wait in a bigger room usually for 1-3 hours until you hear your number called. And that's the most frustrating part. You have to listen for your 10 digit number to be called in rapid-fire Spanish over an intercom speaker by a guy who's clearly bored with his job. Make sure that you bring your passport.