No person engaged in commerce or food commerce, or both, in the course of such commerce, shall discriminate in favor of one purchaser against another purchaser or purchasers of a commodity, or service, or output of a service trade, bought for resale, with or without processing, by contracting to furnish or furnishing, or by contributing to the furnishing of any services or facilities connected with the processing, handling, sale or offering for sale of such commodity, or service, or output of a service trade, purchased upon terms not accorded to all purchasers on proportionally equal terms.