It is not possible to correct COGS retroactively on a shipment that has already been fulfilled as it is recorded at the time of shipment. To update this value, you will have to delete the shipment and re-create the shipment to record the COGS with the updated MAC. The new shipment will have the updated COGS value.
Please do keep in mind that this will send the shipment out to your Accounting Integration as a new Shipment at the time of being re-shipped.
