Costco has something like 1% of the product selection that Walmart has.. let alone online purchases... Given, I tend to order a lot via Amazon, but no brick and mortar store comes close to the U.S. market penetration that Walmart has. Most of the population (mid-high 90's iirc) is within 5 miles of a Walmart.

Say what you will, but they're probably about the only shopping choice in competition with the likes of Amazon.