Tax Differences between PO and Invoice

In a meeting today with one of our customers, they expressed displeasure with Oracle Purchasing/Payables because Oracle did not have some way to correct tax differences caused by errors created by incorrect tax rates being calculated on Purchase Orders due to incorrect ship to locations, incorrect tax rates being input into Oracle, etc. I suggested updating the POs to fix the errors so the invoice would match the PO, but this was not considered to be practical because it happened to frequently. This may be a foolish question, but does anyone have any suggestions which might help?

Thank You,