I saw on TV that Costco's most popular item is the TP.
Before Costco, I can't believe the frequency at which I used to have to buy TP and paper towels from the grocery store. And the frequency at which I would forget to buy them, and we would run out. Now, we hardly ever run out of either.
Maybe I like Costco so much because it is an especially good place for scatterbrained people to shop.