CostCo is just a knock off Wal-mart/Sam's Club. (To tie this into Himu's thread)
It's a little bit different.
Costco have about 4000 SKUs in store, where an average Walmart supacentre has 120,000 SKUs.
You want X? Good, come to Costco. Here is X. It is cheap and super good quality. Occasionally, they might have a second option, but their entire cost model is based on creating a loyalty to the Costco brand rather than the products.
I pretty much buy about 25% of my household groceries from Costco now. Things like cheese where we don't really care about the brand but just want something that isn't shit is a great example. Get in, buy cheese, get out. Sorted.
I fucking love Costco.