R.I. Gen. Laws § 44-5-9: Deductions and penalties to insure prompt payment.

Any city or town may provide for a deduction from the tax assessed against any person, if paid by an appointed time, or for the penalties by way of percentage on a tax, if not paid at the time appointed, not exceeding eighteen percent (18%) per annum, as it deems necessary to insure punctual payment; provided, that the city of Cranston may charge a penalty not exceeding twelve percent (12%) per annum.