@Godzgirlwalking I will be making the same trip (San Sebastian to Saint Jean Pied de Port), only a few days before you. After much research, the most sensible way seems to be BlaBlaCar. What would take 4 - 5 hours by train or bus is but a mere 1.5 hours in the car, if one can secure the ride.
That's more time for me to spend in SJPP, getting my official Pilgrim Passport, handing off my luggage to a transfer company, and stocking up on snacks for the long trip to Roncesvalles the next day. And, hopefully, getting to Mass before dinner at my chosen albergue.
Plus, the experience of BlaBlaCar, unknown to so many of us Americans, will make an interesting subject to write about, post-Camino.