Although Ivar's service is wonderful, I use Express Bourricot when sending my extra luggage to Santiago from St Jean Pied de Port. The cost is €70, and the process is much easier than dragging your luggage to the post office to be shipped. You just have to go to Express Bourricot's office, give them your information and €70, and they will pick your bag up at your accommodation. When it arrives in Santiago it is stored in Loop Inn's secure luggage room. The hotel has 24 hr reception so you can pick it up at your leisure.
www.expressbourricot.com
However, if you do want to ship to Casa Ivar the details are here:
Send your luggage to me in Santiago de Compostela
casaivar.com