SDCL § 5-26-3: Interest on overdue payments--Rate.

Any proper invoice not paid within forty-five days shall accrue interest beginning on the thirtieth day after receipt of property or service and receipt of the invoice covering the delivered items or services. Interest shall accrue and be charged on any payment overdue under § 5-26-2 at the Category B rate of interest as established in § 54-3-16 or at the rate specified by contract.