S.C. Code Ann. § 12-6-4920

An interstate motor carrier which within a taxable year (1) owns or rents real or personal property in this State except mobile property; or (2) travels more than twenty-five thousand mobile property miles within this State; or (3) makes more than twelve pickups or deliveries in this State shall file an income tax return and remit the amount of tax due. The provisions of items (2) and (3) of this section apply to the holder of the operating authority issued by the Interstate Commerce Commission, not to the interstate motor carrier's agents.