Mass. Gen. Laws ch. 93, § 43A: Fees, penalties or other charges for sending original bill or invoice by postal mail

Section 43A. A person engaged in trade or commerce shall not assess a fee, penalty or other charge for the sending of an original bill or invoice by postal mail to a natural person in this commonwealth; but this section shall not apply to a reasonable discount offered to a natural person for voluntary agreement to pay or otherwise satisfy a bill or invoice after its receipt by means other than postal mail, or for voluntary options such as customized bills or invoices.