Why not post your excess baggage to Santiago? If you write Peregrino on the boxes and an expected date of arrival (and - as Ivar has informed us, write Santiago as a return address) you can collect it when you get to Santiago.
You will have to show your passport or ID when you collect your boxes so write your name on the boxes as it appears in your document.
Jones M. A.
Lista de Correos
15780 Santiago de Compostela
A Coruña