19 CFR § 145.11 - Declarations of value and invoices.
(a) Customs declaration. A clear and complete Customs declaration on the form provided by the foreign post office, giving a full and accurate description of the contents and value of the merchandise, shall be securely attached to at least one mail article of each shipment, including shipments of special classes of merchandise treated in subpart D of this part. Although a Customs declaration is required to be attached to only one mail article of each shipment, examination and release of the merchandise will be expedited if such a declaration is attached to each individual mail article.
(b) Invoice or statement of commercial value. Each shipment of merchandise shall have an invoice or bill of sale (or, in the case of merchandise not purchased or consigned for sale, a statement of the fair retail value in the country of shipment), giving an accurate description and the purchase price of the merchandise, securely attached to the outside of the mail article or enclosed therein. If the shipment consists of more than one mail article, a copy of the invoice should accompany each mail article, or else the invoice shall accompany the mail article bearing the declaration, and that mail article shall be marked “Invoice enclosed.”
(d) Shipments without declaration and invoice. Shipment of merchandise which are not accompanied by a Customs declaration and invoice in accordance with paragraphs (a) through (b) of this section may be subject to seizure and forfeiture in accordance with § 145.4.