Each person selling or delivering sewing machines, either in person or through agents, shall pay $25 annually to the state for each county in which he may sell or deliver sewing machines. For each motor vehicle used in delivering or displaying the same, an additional license shall be paid to the state of $10; provided, that a merchant carrying sewing machines as a part of his stock in trade and whose principal business is not selling sewing machines shall not be required to pay this license.