All good business sense, the only lag I could see (and it happened to me), is the parts house has raised it's prices and neglected to inform you, thus the caveat, "bill from the invoice". Easier said than done when you want to bill and have not gotten the invoice.
But most of us buy in advance to keep stocked up, so you shouldn't be buying daily and worrying about the daily price. Plus when we buy, we get our invoice right then and there and can see the price. But with my markups, a few percentages won't hurt if we miss a price change.