No agreement, formal or informal, shall require the use of any promotion, premium, coupon, give-away, or rebate in the operation of the business; provided, however, that a retail dealer may participate in a promotion, premium, coupon give-away, or rebate sponsored by a supplier and completely paid for by said supplier, or unless said dealer elects to participate at his own expense.
Mass. Gen. Laws ch. 93E, § 3A