Correos, the Spanish mail service, will hold a box for you for 14 days. If you need longer, you can purchase a 30-day hold or a 45-day hold. The program is called El Camino con Correos. They have a Facebook page, and you can contact them through Facebook Messenger. They respond within minutes in excellent English. At least they did for me.
I sent a box of clothes, etc., to myself from Burgos to Sarria last spring, to be picked up 30 days later. There was a bit of confusion at the Burgos post office, as no one had any idea what I was talking about. And their English was about as good as my Spanish, which is to say, not very.
Fortunately I had saved the Messenger exchanges and the counter employees called over an extremely helpful man, whose English was fairly good. He sorted it all out. The box, which I purchased in Burgos very a very small fee, was there waiting for me in Sarria.
If you use this service, be sure to save your receipt and any paperwork. You'll need it to pick up the parcel at the other end.