Massachusetts General Laws Part I. Administration of the Government (Ch. 1-182) Ch. 105A, § 5

If an occupant is in default for a period of five days or more, the operator may deny the occupant access to the leased space in a reasonable and peaceable manner; provided, however, that the occupant may have access at any time for the sole purpose of viewing the contents of his leased space in order to verify the contents therein. The operator may impose a reasonable late fee on the occupant for each month the occupant does not pay rent when due. For purposes of this section, a reasonable late fee may be computed as the greater of $20 per month or 20 per cent of the amount of monthly rent. Any late fee imposed by the operator pursuant to this section shall be in addition to any other remedy provided by law or contract.