Thanks, mig, I knew there was storage at Barajas but couldn't find info on the website.
One thing I vaguely remember is that there is an approximately 2 week limit on keeping the luggage at the airport. After that, they transfer the bag/s to the warehouse, and the link you posted shows a pretty hefty transfer fee. I don't think that's a voluntary thing, I'm pretty sure that if you leave it longer than the allotted time, they automatically move it to the warehouse and you have to pay the fee. So for people walking the camino, it's a very expensive option.
Bill, I think there's a good chance that if you have reservations in a Madrid hotel for your stay after the Camino, you will be able to leave it there at no charge. I've never had a hotel say no, either in Santiago or in Madrid. And depending on your travel plans, you could either personally take the box to the hotel or just mail it from a post office.
Buen camino, Laurie