Upon cancellation or expiration of collateral protection coverage, the amount of unearned premiums, if any, as calculated in accordance with the policy approved by the department of commerce and insurance as permitted by law, shall be refunded to the debtor. The amount of unearned premiums, however, may not be calculated by the rule of 78 or sum of the digits method. A refund of unearned premiums may be credited to the debtor’s obligation under the credit agreement or distributed directly to the debtor by check or other means.