A client wants to have quantity discounting based on product families as opposed to individual products. For instance if a user bought 10 of one product and 5 of another from the same category/family, they would get quantity discounting based on 15 items.

Does anyone know of an available addon/customization someone has already done for this sort of thing. (Or even if someone did a custom job themselves?) Thinking about it myself, it seems like this would require changes to quite a good deal of the zen-cart files.