Does The Post Office In Spain Offer A Storage Service In Santiago?

[ivar]

Yes they do, have a look here:
http://www.correos.es/ss/Satellite/..._paqueteria/detalle_de_producto-sidioma=en_GB

 

[notion900]

Be aware that the Correos collection office (Lista de Correos) is closed Saturday afternoons, Sundays and public holidays.

If you think you may walk into Santiago on a Saturday, be aware that the Correos office closes at 12.00 and Mass is at 12.00. It is hard to be there early enough to collect your items, check into accommodation and make it to noon Mass.

@ivar offers more flexibility.