Lindsay - here is the information I think you are looking for, it is from
http://www.americanpilgrims.com
Can I mail a package ahead to myself? Yes, and this is fairly common. Many do it after starting out to lighten their load; others may do it with aforethought, sending a package of 'city clothes' ahead to Santiago. The service in Spain, called lista de correos, is the same as poste restante or, in the U.S., general delivery. You can buy a box at the correos (post office). Packages are addressed:
Your name with your surname first and your surname in capital letters and underlined or boxed
LISTA DE CORREOS
Destination city with postal code and province (see below for some)
The Correos will normally hold a package for 15 days after which they will return it to the city of origin. That would be very bad news! It has been rumored that the Correos will hold a package longer than the usual 15 days for peregrinos. Information received by a returning peregrino on June 16, 2008 is that packages are in fact NOT being held longer than 15 days for peregrinos. Be advised! You might write "PEREGRINO" boldly on the box and you might also add "Retener en lista de correos hasta el <day> de <month>" ("Hold until date" with the month spelled out in Spanish) but apparently neither of those markings will gain you a guarantee of any special treatment.
The lista de correos addresses of a few major cities along the Camino are:
Paseo Sarasate 9/31080 Pamplona (Navarra)
Paseo de Inmaculada 5/31200 Estella (Navarra)
Perez Caldo 44/26080 Logroño (La Rioja)
Plaza Conde de Castro 1/09080 Burgos (Burgos)
Jardines de San Francisco s/n/24080 León (León)
Calle General Vives 1/24400 Ponferrada (León)
Calle Calvo Sotelo 183/27600 Sarria (Lugo)
Orfas 17/15703 Santiago de Compostela (A Coruña)
You will need your passport for identification when retrieving your package.
Have a further question about mail service in Spain? Link to the Correos de España web site (click on "English" lower right corner).
By the way, the information for poste restante service in France can be found at the site for Discover France (English). Look for "General Delivery Service." We have seen advisements against sending packages from France (St.-Jean-Pied-de-Port for example) to lista de correos in Spain. Apparently the two systems don't recognize each other.