Notwithstanding the provisions of any other law, the seller or assignee under a revolving charge agreement may charge, receive, and collect a time-price differential which shall not exceed the following rate: One and three-quarters percent per month on amounts less than five hundred dollars, and one and one-half percent per month on amounts of five hundred dollars or more. The rate shall be computed on the unpaid balance under the agreement from month to month, which need not be a calendar month, or other period as agreed, except that no time-price differential shall be assessed on an account if payment of the outstanding balance is received prior to the beginning of the next billing cycle under the agreement.