Hi Anne,
This is NO problem; I've done it every time I've walked the Camino. When you arrive in Ponferrada, go to the Correos, or post office. I'm guessing you will be coming in by bus? The info desk at the station can give you a map and show you where the Correos is. It's about a 10-15 minute walk from the station, close to the old center of town.
At the Correos you will need to first buy a 'Caja Verde' (green box), the supre grande size. The box will cost about €3. Ask for 'sello', which is tape, and you can then assemble the box and drop you suitcase inside. The box measures 590 x 390 x 390 mm, so you can check to be sure your suitcase will fit - I put a large wheeled duffel bag in one once, no problem. The max weight allowed is 20 kg.
Once the box is loaded and sealed, address it as follows:
LAST NAME, first name, Peregina
Listo de Correos
Santiago de Compostela, 15780
(verify the postal code)
Also, write this same information as the return address. The next step is to get a number for the 'Enviar' line, which means 'to send.' The ticket machine will be near the front door of the Correos. At the Enviar desk you will heft the box up onto the counter, have it weighed, and pay for the shipping (around €10). The clerk will give you the shipping form to complete; be sure to keep your copy, as you will need to present it with your passport when you claim your box in Santiago.
The Correos in Santiago will hold the box for two weeks, no charge. That should give you enough time to make your walk.
Another option is to send it to The Pilgrim Helper at Pension Badalada,
http://www.badalada.es. They will hold your box until you arrive for €25.
For this option, address the box as follows and follow instructions above:
Pension Badalada
ATT: [Your name here]
Rua Xelmírez, 3
15704 Santiago de Compostela
Hope that helps! Buen Camino!!!
Nancy