Costco, at least in the USA, is another departure from what we've been discussing here so far. Costco is what I refer to as membership-based wholesale. Basically, there is a yearly membership fee (e.g. US$50) which is required simply to gain entry to the store. The store is very warehouse-y in feel, and everything is sold in large quantities. For example, you can't buy a bag of chips, you buy a BOX of bags of chips. These stores have branched out to electronics and toys and naturally, don't apply the large quantity requirement there.

In the USA, this is the same as BJs and Sams Club (Walmart's wholesaler)