Carry the extra shoes into Spain and post from Pamplona. The Correos will hold packages in Poste Restante (General Delivery) for at least two weeks. There is an urban myth that, if you write PEREGRINO under your address, they will hold it for up to one month. But, no one has validated this yet. Please correct me if you know otherwise for a fact.
Last summer, while I was working at the Pilgrim Office, I asked David from the Correos mini-office there about this. He told me he had never heard of the practice. He knew about the standard two-week hold. That was official policy. But the peregrino thing was news to him.
Another idea that works, especially if you have advance hotel reservations at Leon is to mail the parcel to the hotel with a notation on the outside of the box stating:
See Reservation Number: 34567890, Arriving on da/mo/2016.
In Spanish this would read:
Ver Número de reserva: 34567890, Entrando el 99 / 07 / 2016 (day/month/year).
I have done this and it works. But I cannot say if it works all the time and everywhere. Contact the hotel you plan to stay in to ask if they will hold a parcel for you until you arrive.
I hope this helps.