The Colbert County Commission is hereby authorized to impose excise taxes in an amount not to exceed two cents ($.02) per gallon on persons selling, distributing, storing, or withdrawing from storage for any purpose whatever, gasoline and motor fuel within such counties and to require every distributor, retail dealer, or storer of gasoline or motor fuel as herein defined to pay such excise taxes upon the selling, distributing, or withdrawing from storage for any use, gasoline, and motor fuel as herein defined in such counties; provided that excise taxes levied pursuant to the provisions of this subpart shall not be levied upon the sale of gasoline or motor fuel in interstate commerce, and provided further that if the excise tax imposed under this subpart upon the sale of such gasoline or motor fuel shall have been paid by a distributor or by retail dealer or storer, such payment shall be sufficient, the intention being that the taxes shall not be paid but once. The excise taxes imposed pursuant to the provisions of this subpart shall apply to persons, firms, corporations, dealers, or distributors storing gasoline or motor fuel and distributing the same or allowing the same to be withdrawn from storage, whether such withdrawal be for sale or other use, provided that sellers of gasoline or motor fuel and its substitutes paying the taxes herein provided may pay the same computed and paid on the basis of their sales as hereinafter required, and storers and distributors shall compute and pay such taxes on the basis of their withdrawals or distributions. The taxes herein provided for shall be in addition to any and all excise or other taxes imposed on gasoline or motor fuel or any device or substitute therefor, or on the business of selling, distributing, storing, or withdrawing from storage for any purpose, gasoline or motor fuel as herein defined; however, there shall not be levied any tax upon any gasoline or motor fuel as herein defined when used in essential governmental functions by the State of Alabama or any agency thereof, county governing agencies, municipalities, any other entity as determined by the county governing body to be exempted under existing state statutes, or boards of education. Provided, further, any user exempt from the state gasoline or motor fuel tax is hereby exempt from the tax levied pursuant to this subpart. Provided further, this tax shall not be levied in that portion of House District 18 lying in Colbert County.