Exchange core values should be indicated on all invoices.  Per US customs regulations, the value of the unit must be indicated on a Commercial Invoice, not the value of the transaction. Both the invoice and Customs Invoice must match. The following excerpt is from the Department of Commerce Foreign Trade Regulations:
 
Section 30.1
Export value: The value of the goods at the U.S. port of export. The value shall be the selling price (or the cost if the goods are not sold), including inland or domestic freight, insurance, and other charges to the U.S. seaport, airport, or land border port of export. Cost of goods is the sum of expenses incurred in the USPPI's acquisition or production of the goods. (See § 30.6(a)(17)).
 
Section 30.6(a)(17)
In general, the value to be reported in the EEI shall be the value of the goods at the U.S. port of export. The value shall be the selling price as defined in this paragraph (or the cost if the goods are not sold), including inland or domestic freight, insurance, and other charges to the U.S. seaport, airport, or land border port of export. Cost of goods is the sum of expenses incurred in the USPPI acquisition or production of the goods. Report the value to the nearest dollar; omit cents. Fractions of a dollar less than 50 cents should be ignored, and fractions of 50 cents or more should be rounded up to the next dollar. (i) Selling price. The selling price for goods exported pursuant to sale, and the value to be reported in the EEI, is the USPPI's price to the FPPI (the foreign buyer). Deduct from the selling price any unconditional discounts, but do not deduct discounts that are conditional upon a particular act or performance on the part of the foreign buyer. For goods shipped on consignment without a sale actually having been made at the time of export, the selling price to be reported in the EEI is the market value at the time of export at the U.S. port.