I can speak from experience on this one, which is rare. I owned 2 retail auto parts stores (NAPA, and then independent) for 14 years. If we got a price increase, we could not seperate what was on the shelf from what would be on the shelves at the future new price. In our case, if a company had a price change, for example for brake shoes and pads, they would send out a new price list, both with the new cost and the new suggested retail. We would then go into our computer database and update the cost and retail prices, and from that time forward that is what we would sell it for, whether it was on our shelf or came in the following week. We would drive ourselves crazy if we tried tried to seperate them. There were MANY occasions that the price of an item would go down. In automotive, as a part becomes more popular, many times the price goes down. In that case, those items on the shelf we would "lose" on. So it kind of all works out in the end. Adjusting the price by piece sounds easy, but in the overall scheme of things it is a lot of labor and much better to bite the bullet and move on with the new price. Now a days, the price changes are generally done online right from the manufacturer, so there's even less opportunity to adjust for whats on the shelf.

Mike