Here is some more useful (I hope) information.
Everytime I go to Santiago I end up buying stuff for myself and friends. Typically, I have mailed a box through the Spanish Correos (very efficient) to be at Santiago when I arrive. See the Forum store (click on "forum Store" link at top right of this page). Ivar received parcels sent from anywhere and holds them for a fee. The main post office in Santiago on Rua Franco received parcels ,mailed through the postal system and holds them for free for 15 days, after that, there is a nominal daily charge.
I usually send a resupply of my nutritional supplements, medications, and some clean "touristy" clothes, along with any souvenirs I bought at SJPdP. BTW, a local industry is making real Basque espadrilles. They are always popular. Prices are VERY reasonable, they are made by local folks, locally, and are available in several shops in town. this is another reason why I always advise people to stay two nights before starting out. SJPdP is a marvelous and scenic town. You should enjoy it.
If you will walk from Saint Jean Pied de Port to Santiago, seriously consider sending a piece of your own luggage ahead using Express Bourricot. This service is not cheap, costs €70 to send one item. But, if walking with a friend, you can share one piece of luggage and perhaps place a folded soft duffel bag inside. I use it every time I start at Saint Jean Pied de Port (aka SJPdP).
Each week, Express Bourricot drives its van direct to Hostal / Hotel La Salle in Santiago (a two-day drive). There, your bag is placed into a locked storage room until you get there. The Hostal offers a discount to folks who use this service. Once you send the bag via Express Bourricot, make a reservation using your smartphone. This place is very conveniently located at Santiago.
Here is the web page for Express Bourricot:
http://www.expressbourricot.com/
And for the Albergue / Hostal La Salle:
http://www.hostallasalle.com/
Hope this helps.