Parallel to that, I have to imagine Costco makes a lot of money off of impulse purchases, which are induced by uncertainty in the specific items they will have available, plus mutable store layouts.

Long ago in undergrad I took a retail marketing class and we did a field trip to Costco; the GM told us it was part of their policy to rearrange parts of the store occasionally so that you had to browse the entire place to check off your shopping list. This increases the likelihood that you stumble across new products. So it’s this combination of “best price/quality without decision fatigue” plus some impulse buying that works for them. The fact that they are figuring out the price/quality trade off for you up front probably also makes it easier to impulse buy with fewer regrets.