An idea regarding charitable giving

Today I had an idea regarding charitable giving that as far as I know hasn't been implemented, at least on a large scale: Why not have "charity cards", credit cards which automatically donate a certain percentage  of what you spend to charity whenever you make a purchase? For example, if you spend $100.00 on something and have a 2% threshold, that would automatically generate a corresponding $2 donation to a charity of your choice. I think that would be good because such "microdonations," which most of us wouldn't bother to make separately, would eventually ad up. They would also require no special effort, which I think is a great psychological  barrier to giving. If a lot of people ended up using charity cards, it could easily generate millions (or more) in donations.  So instead of getting "points" or filer miles, why not have cards that automatically generate microdonations?