How do they get the inventory of a costco if it isn't officially sanctioned by costco? Do they buy the stuff on their corporate card then charge you on yours an

How do they get the inventory of a costco if it isn't officially sanctioned by costco? Do they buy the stuff on their corporate card then charge you on yours and bank off the $10 fee? I don't live in an instacart location, so am not sure how their biz model works (sounds like peapod).