Hola Elin and welcome to the forum.
We too have walked from Santander and then returned there. If you are staying in a hotel/pension at the start and the end then they may keep a packed bag/package for you. It would be worth asking, maybe by e-mail before you leave home. For places to stay you could try booking.com or venere.com, then check the hotels own websites etc. The phone numbers given on both sites are their own only.
Personally we have stayed at the Celuisma near the ferry port, railway and bus stations, but it is not cheap although breakfast is included.
As an extra thought:- to avoid the industrial area between Santander and Torrelavega you might like to think about taking the FEVE cercania to Barreda (approx 8kms then to Santillana del Mar, albergues etc) and then walk your 7 days. You could still be on the FEVE line to get back to Santander, depending on how fast you walk and how far you get.
Buen Camino