Sounds like you need some custom logic or a custom control. You could clone the shoppingcartitemselector control and the modify your control to include the conditions applicable. The only down side could be that you may need to clone multiple controls depending on what you need to do. The shoppingcartitemselector control contains multiple other controls,but I think you could get by with just cloning the main one. Or like Brenden suggested having an event handler that maybe overrides the adding an item to the cart or creating the cart item. Depends on if you want the discount to be visible right away, or later in the process.