I used bag transfer a few times last year during the mini heat wave, there are at least two private companies operating, sorry I can't remember their names, they both seemed reliable, had printed labels for you to fill in etc.
Lugo public albergue seems not to accept bags, they go to a bar a few streets away where they get put in the cupboard under the stairs.
There is no need to book with the private companies, just pick up a label at the desk the night before, there is a little envelope attached to the label, you put in EUR3 I think it was.