An export tax of one-twentieth of one cent (1/20 of 1¢) per gallon is levied upon all of the petroleum products, subject to the special privilege tax provided at §67-3-203, which are stored in this state, or have come to rest after shipment in interstate commerce and are stored in this state, and are subsequently exported to points outside of this state. If with respect to these petroleum products the special privilege tax has already been paid, then nineteen-twentieths (19/20) of the special privilege tax may be credited on a monthly return, or in the alternative, refunded.