Why is it appropriate to include only the incremental cost when estimating the holding and order cost for a firm?

1.Consider a supermarket deciding on the size of its replenishment order from Procter and Gamble. What cost should it take into account when making this decision?2.When are quantity discounts justified in a supply chain?3.Why is it appropriate to include only the incremental cost when estimating the holding and order cost for a firm?