S.D. Codified Laws § 54-11-6.2
A charge may be made in each billing cycle which is a percentage of an amount no greater than the average daily balance of the account, the unpaid balance of the account on approximately the same day of the billing cycle or the median amount within a specified range within which the average daily balance or the unpaid balance of the account, on approximately the same day of the billing cycle, is included. A charge may be made pursuant to this section only if the same charge is applied to all balances within the specified range and if the percentage when applied to the median amount within the range does not produce a charge exceeding the charge resulting from applying that percentage to the lowest amount within the range by more than eight percent of the charge on the median amount. This charge may be compounded.
SDCL 54-11-6.2