I can confirm QKO's assertion about WM and the way they deal with suppliers. I used to work with a major supplier to WM and they do start out by setting a price point they will buy at. Suppliers will often agree even if they lose $$ or make very little hoping that they will eventually be able to increase the price. When they realize that they will not be able to increase price (indeed WM will begin pressuring them to decrease almost immediately), they either have to find a way to manufacture more cheaply or stop supplying.
The problem with discontinuing the relationship with WM is that WM may very well stop carrying all products by that supplier. So there is tremendous pressure to find a way to supply at a lower cost and that usually means a decrease in quality to survive.
I am not a WM hater but it wields a lot of power in the marketplace. Most of the time, there is not a similar level of power to its suppliers.