An intriquing question, postal services worldwide have become, well, snail mail is the best description without becoming profane, and is often slower or worse. French Poste restante will keep a letter for 15 days before returning - concerning parcels I have no idea but doubt if at all possible. There is also the Chemin to confront - 560+ kilometers as the crow flies-not including diversions and while not especially arduous the first half will have some strenuous surprises if you have not walked there before and who knows, there is always unexpected to encounter on any Camino, those for good and those for less then good which may include, GF, injury, sickness, or ennui. I would consider buying there - France is by all accounts now a civilized country, much less hassle, no worry, nothing lost, probably cheaper then a parcel service, and should your purchases not be particularly appreciated, there is always a nearby albergue which will be happy to take your discards.