How about maximum one (greatest value) listed? That would eliminate multiple links. I think it's difficult to make moral judgements about the businesses, but you could exercise some control over the wording on the link.
Isn't it the case that some of these non e-learning donations are being made on the basis that their link will appear, and therefore gain Google points?
I guess any serious / sincere donator would be happy with their name in standard text (not a hyperlink to their website).