Section 9. Except as otherwise provided in section seven, the tax in every instance shall be borne by the purchaser, and no person offering fuel for sale shall sell, advertise or offer for sale said fuel separately from the tax imposed by this chapter. For any violation of this section, the license to keep and sell crude petroleum or any of its products, issued by the licensing authority under chapter one hundred and forty-eight, shall be suspended by the said authority on request of the commissioner of revenue for such time as said commissioner deems proper.