Marc, you know I would just roll in to a charity shop with a box of cards, nicely packaged in cello bags with perhaps a 'handmade for Charity X' label inside, and ask to see the manager there and then. They will be gratefully accepted I am sure.
I do avoid Christmas cards as my particular charity does remakes; patients in the hospice make them to help fundraise.
How about a selection of birthday cards, cards for men, animal pics, and some really nice Thinking of You cards?
To hell with selling cards for 50p: the feeling of contributing to a good cause will far outway that.
Good luck and enjoy your cardmaking.
Judy C