I feel for you trying to get parcels, letters and packets in and out of Spain. Correos express is NOT the same as the normal Correos post office.
I could fill this whole forum with dire tales of things that disappear never to be seen again. I can't find a good word to say about a single carrier in Spain.
I have now given up trying to get things sent to my home address and now have them sent to a bar in town. It think the carrier is afraid to come out into the campo. Here be dragons!!
To improve your chances always send stuff registered to be signed for.