Actually I didn't know they had the to-and-from package. I saw that the Santiago post office had cardboard shipping boxes for bikes and that they had special services for pilgrims so I did a search of the correos site.
I bet that the shipping from France would be really expensive. Maybe it would be cheaper to go to Pamplona and ship your case from there and then bus with the bike to Roncevalles, about 12 euros for both you and the bike. See
http://www.autocaresartieda.com/?scc=roncesvalles
You could then start from there or, if you really want to start from SJPdP, ride down through Valcarlos and then back to Roncevalles via the Route Napoleon. I suggest going down via Valcarlos instead of up as there is traffic and it can PROBABLY be handled better going faster as there would be fewer cars passing you.
You might also be able to take your bike with you on the ALSA/CONDES bus from Pamplona to SJPdP but it is a more expensive trip for you and I don't know about the bike.
I see a 2017 Correos' PDF page that looks like they will send a boxed bike up to 20 kg one-way for about 42 euros or less (zona 2 or zona 3) so an empty case might go for less (if the dimensions are about the same as their box.) The PDF file was found at:
http://www.correos.es/ss/Satellite?...goBlobs&blobwhere=1365529581987&ssbinary=true
or click the PDF symbol labelled "Tarifas Paq Bicicleta" on this page:
http://www.correos.es/ss/Satellite/...roductos_az/detalle_de_producto-sidioma=en_GB
A word of caution. I shipped items ahead in Spain and we ended up going very slowly so I had to contanct Correos a few times along the way so they would hold our stuff (I didn't know about Ivar's service.) Count on them not speaking English. If you don't have a translation app recruit a pilgrim who can help you.