good day tutor please help and please base on customs law of Philippines

The buyer purchases and imports housewares from the Seller for a price of $10,000.00. The imported housewares turn out to be defective. Consequently, the Seller grants the Buyer a $5,000.00 credit. On the five (5) subsequent importations of identical goods, the buyer adjusts the value of each shipment down to $9,000.00 as compensation for the duties already paid due to the "overvaluation of the defective goods at $10,000.00

The Buyer explains to you, a customs officer in the Philippines that the negotiated price credits on the five (5) subsequent shipments from the Seller. The purchase order for the five (5) subsequent shipments do not reflect the negotiated credits and therefore contain price quotations that are higher than the invoice prices. Nevertheless, the Buyer deducts the credited amounts from the higher figures reflected on the purchase orders and pays the lower invoice price.

Question:

a. How do you value the imported goods? Explain.