Shipping stuff downstream to Ivar is one of the best services you will pay for. Shipping from France into Spain is about double the cost of shipping from the first post office (Correos) in Spain (likely Pamplona). However, it is not onerous. Shipping parcels within Spain is a bargain.
On both my
Camino Frances journeys (2013 & 2014) I sent things from St. Jean Pied de Port to an albergue in Santiago to await my arrival, a month later. I used
http://www.expressbourricot.com/. Caroline is excellent to deal with. She operates a baggage and near-distance person transport service for pilgrims starting at St. Jean Pied de Port.
Basically, Express Bourricot will send one piece of luggage (subject to size and weight limits) to an albergue at Santiago with a secure baggage storage room. They send a van perhaps once a month, or whenever they have enough luggage to justify the expense of sending a driver on an overnight drive to Santiago. That's a four-day round trip, and she is trying to run a business. Just the fuel for the journey has to be a HUGE expense.
This service is necessarily expensive (@ €70 for one piece of luggage in May 2014). However, it is worth it if you want to send "tourist clothing," shoes, toiletries, your rucksack shipping bag, and any souvenirs you bought in France, ahead of you. They are reliable, honest, secure, and trustworthy. IMHO, it is cheaper to send the one piece of luggage, especially if two people share it, than it is to ship multiple parcels from the post office at St. Jean. I have done it twice now, and would definitely do it again.
Once I send the piece of luggage from France, I use the Spanish Correos exclusively to send stuff to Ivar. I usually send excess gear, tourist stuff I accumulate along the way (books, pamphlets, museum guides, postcards) souvenirs, and other purchases I will not need until Santiago. I do not pass over something clever or a good deal just because I am on pilgrimage. I leverage my pilgrimage to address all my varied interests. It is all about the journey...
Anyway, you can send about two kilos to Ivar for less than €10 from just about any Correos. There are three size boxes you will want to consider: Mediana, Grande, and Extra-Grande. There is one smaller size, about the size of a thick novel, but I advise against it as it holds so little. The Mediana Cayo Verde is about the size of a large shoe box (think mens shoes). It holds about two to three kilos of small stuff. The boxes are (or were) green and white, hence the name Cayo (box) Verde (green).
At the Correos, you buy the box, fill the box, and mail the box. This involves two transactions, the first to buy the box and get the forms, and the second to weigh and post the box. the instructions are self-explanatory. Save the paper receipt you get. I always send a copy of the receipt that I photographed using my iPod Touch to Ivar to let him know what is coming, and to him provide the ability to track a package should it "go walkabout" in the system. Hey, life happens...
The best thing about Ivar's kind service is that, if you write in your e-mail address on the bottom of the return address you used on the shipping label, Ivar will send you an e-mail once he receives your parcel(s). The Correos will NOT do this. Also, Ivar will hold stuff for six months, where the Correos will hold things for 15 calendar days.
In this regard, there is an urban myth about the Correos extending this to 30 calendar days if you write "PEREGRINO" in block letters so the receiving post office at Santiago sees it. However, I have yet to see or read confirmation of this as a de facto Correos policy.
Finally, on my first Camino I sent three Mediana boxes containing about 8 kilos in total to Ivar. On my second Camino, and clearly not having learned anything, I sent 11 kilos in three Mediana boxes and one Extra-Grande box to Ivar. Of course I asked him for the "Pilgrim Idiot's" volume discount. He is a gentleman. We made a joke out of it and he kindly discounted his single parcel charge. But, again, IVar has a business to run and this does provide a HUGE convenience to pilgrims heading for Santiago.
You would not believe the stuff people send Ivar. I have seen bicycles in cartons and shrink-wrapped, shrink wrapped and cartoned luggage, and all manner of boxes and sacks. Actually I was surprised that the Correos will accept these items for shipping. But, live and learn. I recall even seeing a baby car seat that was shrink-wrapped at an airport.
All of the information you need about sending stuff to Ivar can be found on this forum.
I hope this helps.