Section 4. At the time of filing a return required by section sixteen of chapter sixty-two C, every licensee other than a user shall pay to the commissioner an excise at the tax per gallon determined by the commissioner under chapter sixty-four A upon each gallon of special fuels sold or used by him in the commonwealth during the calendar month covered by the return. Notwithstanding the foregoing, the tax per gallon payable upon each gallon of liquified gas shall be separately determined by the commissioner, utilizing the same procedures as those used for fuel under chapter 64A, at a rate of 19.1 per cent of the average price computed to the nearest tenth of one per cent per gallon and such tax per gallon as so determined shall apply to each gallon of liquified gas sold or used by a licensee in the commonwealth during the calendar month covered by the return.