The parcel is now safely in my hands and will wait for
@Fred Bovenkamp at my place. If you run into similar problems, the solution is:
Find "somebody" in Santiago that will pick up your parcel.
Ask them for their full name, address and passport number.
Send a letter of authorization (in Spanish!) to that person in Santiago, with your name, your passport number, the tracking number of your parcel AND a photocopy of your passport plus the info mentioned above.
Ideally get a stamp and signature (as witness) from somebody else, f.e. hospitalero.
Done!
Buen Camino, SY
Sample text for said letter replace text in {brackets} with your info:
Estimados Señores y Señoras,
Yo {your name as it appears on the parcel} autorizo a {name and passport number of the person who will pick parcel up}, con domicilio en {address of said person} de recibir el paquete que he enviado el día {date when you send it} desde {town where you sent it from} con el numero {insert tracking number and/or any other information that might help identify the parcel} porque soy peregrino en el
Camino de Santiago y no voy a llegar en tiempo para recogerlo yo mismo.
{Your Name, signed and "printed"}
{Your passport number}