An employee of a “retail or service establishment” who is paid on a commission basis or whose pay includes compensation representing commissions need not be paid the premium compensation prescribed by section 7(a) for overtime hours worked in a workweek, provided the following conditions are met:

(a) The “regular rate” of pay of such employee must be more than one and one-half times the minimum hourly rate applicable to him under section 6, and

(b) More than half his compensation for a “representative period” (not less than one month) must represent commissions on goods or services.