There are companies that will transport your bag for you every day, every other day, etc. You don't need to make arrangements ahead of time - simply fill out an envelope, available in all accommodations on the camino, put your euros (usually 5-6), and leave it at the front desk. Your bag will be at your designated destination when you arrive. These services are safe and reliable, though you should always carry your valuables with you in your walking pack.
Luggage transfer day to day
average $5.00 a day (give or take a euro). There is no need to book ahead of time.
You can also mail a suitcase or bag from any place along the Camino directly to Santiago de Compostela and it will be safely stored until you arrive. Luggage transfer to Santiago de Compostela direct completely depends on the size of your bag, but I've never paid more than $20 to have it shipped.