Sorry i missed answering this one.
Did you see the feature of Kentico's Order Discount? This can adjust the price based on Percent or fixed, and it has "further conditions" which is a macro.
There's a "Shopping cart contains products" option in the macro generator which allows you to select 1 or more products, so you could add one of these per product to make it truly a single sku item.
Try fiddling around with that, i think that's where you need to go!