Section 21A. Cities and towns may provide by ordinance or by-law for the licensing of persons to keep open their places of business for the retail sale of beverages derived wholly or in part from cereals or substitutes therefor and containing less than one-half of one per cent of alcohol, unfermented grape juice, ginger ale, root beer, sarsaparilla, pop, artificial mineral waters, carbonated waters or beverages, natural fruit juices, and other so-called soft drinks, and may fix the fee for said licenses within the limit hereinafter provided, except that in cities having licensing boards the authority to provide for the licensing of such persons and the fixing of fees therefor shall be vested in said licensing boards. For the purposes of this section, the term retail sale shall mean the providing of a beverage as herein defined to a patron in or by a licensed premise in exchange for something of value by means of an in-person exchange or by means of a vending machine exchange or by means of a cover charge, so-called, which gives the patron the right to obtain drinks without additional payment.