Mont. Code Ann. § 31-1-241

31-1-241 . Finance charge limitation. (1) The finance charge included in a retail installment contract must be at a rate agreed upon by the retail seller and the buyer, but the finance charge may not exceed 36% per annum.

(2) The finance charge included in a retail charge account agreement must be at a rate agreed upon by the retail seller and the buyer, but the finance charge may not exceed 36% per annum.

(3) The finance charge must be computed from month to month (which need not be a calendar month) or over another regular billing cycle period by using either:

(a) the average daily balance in the account in the billing cycle period; or

(b) the ending balance of the account as of the last day of the billing cycle period less the amount of total purchases charged to the account during that billing cycle.

(4) A seller may change the terms of a revolving charge account whether or not the change is authorized by prior agreement. The seller shall give the buyer written notice of any change at least 45 days before the effective date of the change.

(5) If the retail seller increases the finance charge on a retail charge account agreement, then the increased rate may only be applied to the balance consisting of purchases on other charges incurred on or after the effective date of the increase.

(6) For purposes of determining the balance to which the increased rate applies, all payments may be considered to be applied to the balance existing prior to the change in rate until that balance is paid in full.

(7) If the finance charge determined pursuant to subsection (3) for a monthly period is less than 50 cents, a maximum finance charge not in excess of 50 cents may be charged and collected for the period.