Mich. Comp. Laws § 328.226

Sec. 16. (1) A prepaid contract that is not in writing is voidable by any party to the prepaid contract. (2) A prepaid contract made in violation of this act or with a person who is not registered pursuant to section 6 is voidable by the contract buyer or by a personal representative of the contract beneficiary. (3) The parties to a prepaid contract may agree in writing for the payment of a rate of interest not to exceed 10.5% per annum with the following conditions: (a) A prepaid contract made under this act shall not provide for a rate of interest added or deducted in advance. Interest shall be computed from time to time only on the basis of unpaid balances. (b) A prepaid contract made under this act shall not provide that the rate of interest initially effective may be increased for any reason. (c) A contract seller shall not impose any fees or charges in addition to interest in connection with the financing of a prepaid contract. (4) A contract seller shall not offer financing or offer to obtain financing of a prepaid contract under terms and conditions other than allowed by this section.