You don't want the "buy one get one free" to apply directly to the cart subtotal. You want to show the customer which entry is free (or discounted), so they can see that the rebate is applied correctly.

You probably want to check all 'rebate rules' attached to all products in the cart whenever one is added/removed. An active rebate will then modify the price of the cart item, and the sum method can be stupidly simple.