The math isn't working properly in my form

I have a form where there are two different items that can be selected in multiples, and then four different choices of warranty at four different prices that can apply, based on which item and how many of them. It works beautifully when adding them together, but if you delete one item it does not subtract the warranty price for that one. If, however, you click on the default warranty (which is included) then click again on the desired warranty, it updates. Is there a way to get it to update without going back to the default?