It sells a lot of large things in its warehouses that are expensive to ship. I'm sure you can buy a 50 pound bag of dog food or a giant pack of paper towels online, but will you beat Costco's price? Doubtful.
mostly because we value our own time below the market price to have someone else do it for us. If shipping cost ever plummets then this upend warehouse style stores.
Not just that - if you can get an entire shopping trip worth of stuff in one trip to a big warehouse-store, toss it all in your trunk, you don't need to fit it all in standardized boxes with bubble-plastic padding, and the distance from warehouse to your house will be covered in the most direct route. Even if you valued your own time at market rates, it would probably take less of your time than it would of someone else's, because shipping has more intermediate steps.
