I think in both cases the supplier would get significant lower margin as a result of providing higher quality goods. Supplier cave to the market power of Walmart and Costco. And Walmart and Costco choose to whether they provide lower prices or better qualities. And they maintain this market power exactly by giving more of the surplus to the customers. In the apple case, the brand and eco-system alone can provide that market power, e.g. people love apple products, thus they can pocket the surplus while still maintaining the position.