Mailing from a post office to another, addressed to yourself as "Poste restante" is technically possible.
But frankly? Not such a good idea. Its not widely done and things can go wrong. But more importantly, you are giving away that unique and precious freedom of walking completely detached of time pressure and possessions, –all you need and have is in your bag–. Can't explain/describe this feeling very well, but I would never give it away for some toiletries!
There is nothing that you cannot buy or replenish on the way, so why bother? You'll laugh at the idea once you casually stroll through the shopping streets in Pamplona, Logroño, Burgos, Leon, etc. etc. Carry light, in every sense of the word.
Buen Camino.