When I walked the Camino the second time I used Poste Restante. Here is the info from wikipedia. I put my back pack in a duffle bag and shipped that bag with some clean clothes to Santiago. The issues are you must find a post office to mail it from (i shipped it from Pamplona and already knew from my first Camino where the post office was) and it will also have to arrive on a day when the post office is open for you to collect it which probably eliminate weekends and you should check for local holidays too. I bought a brown paper grocery bag and packing tape and a mailing label with me to expedite things in Spain. You may also want to try contacting a hotel in Burgos if you are staying there and sending it to them which would eliminate having to collect it at the end from another location.
Mail can also be received at Spanish Post Offices.
[22] There is normally no charge for the service. Poste Restante letters should be addressed Lista de Correos, followed by the address of the post office (including the post code, town and province). Put your surname in capitals. An example Lista de Correos address is:
John SMITHLista de CorreosPl. Rosa dels Vents 946730 GandiaValenciaSpain
To find the address of the Correos nearest to you check at
www.correos.es. Click on the
Oficinas link for the Localizador de Oficinas.
When collecting mail from the post office you will need to show some form of identification such as a passport or photo driving licence. Specify that you are collecting a Lista de Correos item as these are stored separately from the other stored mail. The Lista de Correos mail is filed alphabetically and it may be worth asking them to check if an expected package was filed under your first name if they cannot find it under your surname. Post offices vary in how long they will hold your mail.
When addressing mail for Spain, always ensure there is a return address on the parcel or packet.