The best option from SJPdP is Express Bourricot. At €70 to send one suitcase it may seem a little expensive, but it's actually cheaper than transferring the luggage every day until Pamplona and mailing it from there, and much less hassle.
It's also super convenient. You just go to Express Bourricot's office, fill out a form, pay the fee, and let them know where you will be spending the night. In the morning simply leave your bag at your accommodation and Express Bourricot will pick it up.
In Santiago it is held at the Loop Inn hotel which has 24/7 reception, so you can pick up your luggage at your convenience. They also offer a discount on a room for those who use Express Bourricot.
Full info here:
www.expressbourricot.com