Section 10. Except as otherwise provided in section five, the tax in every instance shall be borne by the purchaser, and no person offering special fuels for sale shall sell, advertise or offer for sale said special fuels 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 for such time as said commissioner deems proper.