I think that you'll find that when you add the daily transport cost until Pamplona, the cost to mailing the bag from Pamplona to Santiago, and Ivar's storage fee that Express Bourricot's fee of 70€ seems very reasonable!
First you will have 3 or 4 days until Pamplona at the cost of 7€ a day until Pamplona (actually Express Bourricot charges 8€ per day for the stages between SJPDP and Roncesvalles) , which will between 22€ and 30€, depending on whether you take 3 or 4 days.
Then there is the cost to send it to Ivar, which I'll estimate at 20€ (based on the price I paid to send a small suitcase from Irun to Ivar)
Then there's Ivar's storage fee which is between 20€ and 25€ and you're between 62€ and 75€, plus the hassle of going to the post office during operating hours in Pamplona. And don't forget that once you are in Santiago you can pick up your bag at your convenience when you use Express Bourricot.
I'm not knocking Ivar's service at all, but in my opinion, when shipping from SJPDP, Express Bourricot wins out.