I have a similar logistic question. I will arrive in Madrid and take a train to Irun, my starting point. I have a small box containing things I will need after I complete my camino (in 5-6 weeks). The box will probably weigh about 20 pounds, and I will not need the contents until I return to Madrid.
What I am thinking about is, mailing the box from the post office in the airport (if there is one at the airport) and pick it up 5-6 weeks later at the post office in Chamartin train station(again, if there is one).
The last time I was on the camino, I mailed my excess material from Leon to Santiago. The post office was holding it for me until I claimed it. This time, the box will be bigger, heavier, and held longer period.
Does anyone know whether or not this is do-able? Any other solutions that does not cost more than the value of the content?