6. This act shall also not apply to establishments or to separate and partitioned sections of establishments where commodities are, or merchandise is, sold only as accessory to services rendered in carrying out a contract of lease. The Government, by regulation, may determine the cases in which a sale of commodities or merchandise may be deemed accessory to services so rendered. When such a regulation is made, only those classes of sales provided for therein shall be exempt from the application of this act.