Leave a bag in Santiago?

Here's your answer--
http://www.casaivar.com/luggage-storage-in-santiago-de-compostela/index.html

Placing something with Ivar is an easy way to do this. Sometimes, during he peak season, his storage room is full to overflowing. So, and if indicated, other options, just sayin, are to place a bag in consignment at EITHER the Correos or at the Pilgrim Office (P/O).

1. (P/O) When I worked at the P/O last summer (and looking forward to it again in July) pilgrims making the walk from Santiago to Finisterre and / or Muxia would pay €2 per day to store their bags at the P/O. At the time, you went in to the Group Office, just off the entry vestibule opposite the chapel and posed the question: "Consigna?" The staff would write the ticket up and hand you off to a security guard, who would escort you to the consigna location and place your bag. I do not know if the pracitce remains.

2. (Correos) The other idea is to place the bag in storage with the post office (Correos). I forget what they charge or for how long they will hold it. For convenience, there is a Correos branch immediately next to the P/O. It is to the left as you look a the building. Alvaro (mgr), Maria, or Lali will be happy to help you. Tell them Tomas the American voluntario sent you...

I hope this helps.