Hi, Greg,
I always do the same thing. When I arrive in Spain, I pack up the duffel and those few small items (neck pillow, headphones for airplane, etc) and ship them via Correos to Santiago. Ivar has a great service in Santiago -- and Tinca has given you the link in the above post.
This may be way too much overkill, but what I do is collapse a sturdy box and put it flat into my backpack before I leave. I also wrap a lot of box tape around one of my hiking poles, all of which I carry on the plane. I bring pre-addressed sticky labels (note that to use Correos you have to have a return address in Spain, so I always just list the address of my first stop). When I arrive in Spain from the US, I make up my own box and present it to Correos. I started doing that once when I was in a place where Correos didn't have any boxes and it was a huge hassle. I think they do have boxes now, but I have just gotten into this habit, it's part of my pre-Camino ritual, and it usually costs about 8 euros to ship my small box to Santiago.
Buen camino, I would love to hear how you like the Mozarabe from Malaga, it's one of my possibilities for 2017!