SDCL § 54-11-6.4: Minimum charge on monthly bills when service charge less than minimum charge.

If there is an unpaid balance on the date of which the credit service charge is applied and the credit service charge pursuant to this section is less than the minimum charge, a minimum charge not exceeding fifty cents may be made if the billing cycle is monthly. If the billing cycle is not monthly, a minimum charge may be made which bears the same relation to fifty cents as the number of days in the billing cycle bears to thirty.