@biarritzdon Great idea. Perhaps best to start with a branded collection system, the way many charities collect such items here in the US, using the giant mailbox-looking containers as drop points.
Or, perhaps, partner with an existing charity organization in Spain, one that already has in place a motivated volunteer network, physical locations, and existing distribution channels. (Like St. Vincent's in the US.) No need to reinvent the wheel. This lets the focus be on Camino-specific clothing/equipment, and gets the idea into action much faster. The charity would benefit from the increased exposure and the new revenue stream.
What would be especially nice is if the clothing and equipment were sorted and assembled into fully-loaded backpacks and reserved for charitable groups or organizations that bring, say, members of their communities who could not otherwise dream of taking on the expense of the Camino.
"Travel is fatal to prejudice, bigotry, and narrow-mindedness, and many of our people need it sorely on these accounts." - Mark Twain