An invoice credit for an old overpayment: add it back

A shipment of 1,000 water bottles manufactured in Germany is entered at $20.00 per unit at the Port of Philadelphia. The water bottles were shipped from Hamburg to Philadelphia with a shipping cost of $300.00. On the invoice, a credit of $10,000.00 for a previous overpayment by the buyer has been applied to the selling price. The total due on the invoice is $10,300.00. What is the transaction value of the merchandise based on the provided information?