First, grocery stores work on a 3% margin, so if they have a ton of returns on something, it costs them a ton of money from credit card fees and wages, and possibly from losing the cost of some portion of the product.

Second, grocery stores have limited space and are picky about what they'll put in their store. They want quality goods that will sell well, and they don't want to cannibalize other products that would make them more money. So random supplements don't find their way to their shelves.

* All of a single product is from the same vendor. Very often, that vendor is also the manufacturer. (model)

* Grocery stores have limited shelf space, and thus spend time getting to know their products. (model influencing qa)