If memory serves me correct today, Caroline at Express Bourricot can help you move backpacks form St. Jean several days walk into Spain. However, after that, Caroline's service thins out, and you likely need to set something up with Jacotrans. Try this link - the English one:
I recall they are represented just about the entire
Camino Frances. Their "mochila" service simply moves one backpack or even one suitcase from albergue to albergue, hostal, or hotel for a flat fee, per segment/day. Basically, you lock and leave the tagged bag where you slept. Usually at the front desk. While you are walking, the service comes by, collect your bag and take it to your indicated next stop.
While some pilgrims have opinions against doing this, most of us understand there are pilgrims with special needs and abilities. These services are a big help in allowing these pilgrims to do something they might otherwise not be able to achieve. I started referring to these folks as "slackpackers" or "tourigrinos" last year, until I realized the advanced age and physical condition of many of these people. Using a baggage transport service was simply a tool they used to accomplish the primary goal of completing the Camino. Hence, I no longer think the way I once did. Judge not, lest ye be judged...
Finally, Caroline and Express Bourricot offer a service to transport one piece of luggage from St. Jean to Santiago de Compestela. Last year I shipped a duffel bag. This year a standard rolling suitcase. The cost is Euro 70 per bag.
Yes, it sounds expensive until you realize that she only makes the two-day driving trip once or twice a month. The distance is about 500 miles (800 km). Whoever drives the Express Bourricot van with 15-20 pieces of luggage must sleep overnight on the way to and from Santiago. So, Caroline must wait until she has enough bags to justify the costs of making the journey. The cost is far less than sending an equal amount through the post. Caroline has an agreement with an albergue in Santiago to store your luggage in their locked luggage room until you collect it. it works very smoothly.
I usually send more touristy clothes, souvenirs I bought at St. Jean and other stuff I will not want until I arrive at Santiago. This year, I will be a volunteer at the Pilgrim Office in Santiago for two weeks just after my Camino. So, I need things to live there, as well as for playing tourist elsewhere in Europe afterwards.
I hope this helps.