Hi - when we needed to send our bikes back to Germany at the end of the Camino the post office in Santiago was great - we just dropped the bikes of with them and they sent them back for a very reasonable price (much less than taking them on our flight), but that was several years ago. They probably sell boxes too, and there is an office right near the cathedral with very helpful staff. I have also gotten boxes from small bike shops when traveling and needed them for a flight, ones they were finished with but still good enough to be used.