You don't need to pay to join a warehouse store to save big on bulk purchases. Just order case quantities of canned tomatoes, peanut butter, and similar items. Most stores will give you 10% or more for doing this. Some stores apply the case discount to the sale price.
A case sounds like a lot, but a case of Muir Glen tomatoes only contains six cans. The fall shopping lists in Wildly Affordable Organic calls for nine cans of diced tomatoes and fifteen cans of crushed ones. This means getting a case discount is possible even if you only have a few weeks of grocery money available at a time.
Remember, WAO costs don't assume you get case discounts. They count as extra savings.