Make it possible to apply a discount to groups of products in the cart, based on division by a specific number.
Example: A "buy 3 for $250" deal where:
* If you have 3 in the cart: you pay $250
* 6 in the cart: $500
* 9 in the cart: $750
* Etc.
In addition, it will be smart enough to leave the remainder of items at normal price. For example, if you had 5 in the cart, you'd get $3 of them at $250, but the remaining 2 would be full price.
This is different from volume discounts, which can only have a finite number of quantity breaks.