In the past you could send packages by boat quite inexpensively but they have changed that in the last year or so. Airmail would be dreadfully expensive for anything of substantial weight. By boat, the mimimum price is something like 2400 pesetas for the first kilo and 250-400 per kilo after that. So it would be wise to send one large package rather than sending a few smaller boxes. Go to Cibeles (the Palacio de Comunicaciones metro Banco de España) and you'll have to go around back (to the right). There's a street with doors marked with letters. If I'm not mistaken, it's the door marked N. They will prepare your package, wrap it, and tie it up all for a small fee (250-400 pesetas depending on size). Then, you go to another counter in the same room and that is where you send it from. A package to the U.S. takes anywhere from 3 weeks to 3 months. For Australia I couldn't tell you. By the way, they no longer have a special book-rate
!