Matching rules used for Bulk Pos

We have recently started using "bulk" Pos. we create one Po for the month with one line item and a large quantity. Then receive multiple shipments against it. Our item price can change during that time frame and the Po price gets updated. We get an invoice for each shipment. We don't want to end up overpaying the supplier , so we have a custom rule that looks at the extended amount on the voucher and compares it to the Po price and Receipt quantity. if it is over or under by $100, the