The Lauderdale County Commission is hereby authorized to levy, in addition to all other taxes imposed by law, an excise tax on persons selling, distributing, storing, or withdrawing from storage for any purpose whatever, gasoline and motor fuel within Lauderdale County at a rate not to exceed the tax rate levied on the persons selling, distributing, storing, or withdrawing from storage for any purpose whatever, gasoline and motor fuel within the City of Florence, Alabama, and to require every distributor, retail dealer, or storer of gasoline or motor fuel as herein defined to pay such excise tax upon the selling, distributing, or withdrawing from storage for any use, gasoline and motor fuel as herein defined in such county; provided that the excise tax levied pursuant to this part shall not be levied upon the sale of gasoline or motor fuel in interstate commerce, and provided further that if the excise tax levied pursuant to this part upon the sale of such gasoline or motor fuel shall have been paid by a distributor or by retail dealer or storer, the payment shall be sufficient, the intention being that the tax shall not be paid but once. The excise tax levied pursuant to this part 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dealers, or distributors storing gasoline or motor fuel and distributing the same or allowing the same to be withdrawn from storage, whether such withdrawal be for sale or other use; provided that sellers of gasoline or motor fuel and its substitutes paying the tax herein authorized 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 tax on the basis of their withdrawals or distributions. The tax herein authorized 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, the tax shall not be levied by the county commission upon any gasoline or motor fuel as herein defined when used in essential governmental functions by the State of Alabama or any agency thereof, or county governing agencies, municipalities, and boards of education.