A purchase order is not free, and neither is a case that sits in the warehouse. Order every week and the PO bill dominates; order a year's worth and you are renting space for flour. The Harris lot is the quantity where those two annual bills are equal — here 800 cases, twelve receipts a year. The U is flat near the bottom, so a truckload that lands close to Q* is close enough; doubling the lot is not.