19 CFR § 24.3 - Bills and accounts; receipts.
(a) Any bill or account for money due the United States shall be rendered by an authorized Customs officer or employee on an official form.
(b) A receipt for the payment of estimated Customs duties, taxes, fees, and interest, if applicable, shall be provided a payer at the time of payment if he furnishes with his payment an additional copy of the documentation submitted in support of the payment. The appropriate Customs official shall validate the additional copy as paid and return it to the payer. Otherwise, a copy of the document filed by the payer and the payer's cancelled check shall constitute evidence of payment.
(c) A copy of a Customs bill validated as paid will not normally be provided a payer. If a bill is paid by check, the copy of the Customs bill identified as “Payer's Copy” and the payer's cancelled check shall constitute evidence of such payment to Customs. Should a payer desire evidence of receipt, both the “U.S. Customs Service Copy” and the “Payer's Copy” of the bill and, in the case of payments by mail, a stamped, self-addressed envelope, shall be submitted. The “Payer's Copy” of the bill shall then be marked paid by the appropriate Customs official and returned to the payer.
(d) Every payment which is not made in person shall be accompanied by the original bill or by a communication containing sufficient information to identify the account or accounts to which it is to be applied.
(e) Except for bills resulting from dishonored payments (e.g., a check or Automated Clearinghouse (ACH) transaction), all other bills for duties, taxes, fees, interest, or other charges are due and payable within 30 days of the date of the issuance of the bill. Bills resulting from dishonored payments are due and payable within 15 days of the date of the issuance of the bill.