From SJPdP it's actually best to use Express Bourricot to send luggage to Santiago, where it's held for you in a secure storage room at the Loop Inn.
I have used this service 3 times, and it's so much easier than lugging your suitcase to the post office, preparing it to be mailed, etc. (and it's
@ivar approved for sending from SJPdP to Santiago
) All you have to do is go to the Express Bourricot office near the Pilgrim Office, give them your information and pay their fee. They will give you a tag for your luggage, and then you just leave it at your accommodation in the morning when you start out on the Camino.
www.expressbourricot.com