Any person, not licensed as a receiver, distributor or supplier, purchasing fuel or motor fuel as to which there has been no charge made to him of the tax imposed by Section 2 or 2a, or both, shall make payment of the tax imposed by Section 2a of this Act and if the same be thereafter used in the operation of a motor vehicle upon the public highways, make payment of the motor fuel tax computed at the rate prescribed in Section 2 of this Act on the amount so used, such payment to be made to the Department not later than the 20th day of the month succeeding the month in which the motor fuel was so used.
     This Section does not apply in cases of such use of motor fuel which was obtained tax-free under an official certificate of exemption mentioned in Sections 6 and 6a of this Act.