(a) The county commission may 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 or motor fuel within the county at a rate not to exceed two cents ($.02) per gallon, and require every distributor, retail dealer, or storer of gasoline or motor fuel to pay the excise tax upon the selling, distributing, or withdrawing from storage for any use, gasoline or motor fuel in the county. The excise tax levied pursuant to this part shall not be levied upon the sale of gasoline or motor fuel in interstate commerce. If the excise tax levied pursuant to this part upon the sale of gasoline or motor fuel has been paid by a distributor or by a retail dealer or storer, the payment shall be sufficient, so 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dealers, or distributors storing gasoline or motor fuel and distributing or allowing the gasoline or motor fuel to be withdrawn from storage, whether the withdrawal is for sale or other use, provided that sellers of gasoline or motor fuel and substitutes paying the tax, may pay the tax computed and paid on the basis of their sales as required, and storers and distributors shall compute and pay the tax on the basis of their withdrawals or distributions. The tax levied shall be in addition to any and all excise or other taxes imposed on gasoline or motor fuel or any device or substitute, or on the business of selling, distributing, storing, or withdrawing from storage for any purpose, gasoline or motor fuel. The tax shall not be levied by the county commission upon any gasoline or motor fuel when used in governmental functions by the State of Alabama or an agency of the state, county governing agencies, municipalities, and boards of education.