(a) The monthly ceiling may be used as an alternative to the weekly ceiling only for a contract that:
(1) provides for a variable rate, including a contract for an open-end account; and
(2) is not made for personal, family, or household use.
(b) A contract that provides for the use of the monthly ceiling may not provide for the use of another rate ceiling provided under this subchapter.
(c) If the parties agree that the rate may be adjusted monthly, they may agree that the rate from time to time in effect may not exceed the monthly ceiling from time to time in effect, and the monthly ceiling is the ceiling on those contracts.