I don't know the answer to your question, but isn't Instacart free to resell anything they bought at Costco? So would they even need an agreement from them?
EDIT: It's not. I somehow assumed it would be. Interesting to know it isn't.
That being said, the reselling is normally for people who do something with it (ie cook the food, print with the ink, etc), not turn around and sell the exact same product.
I'm guessing Costco's business is at least somewhat dependent on having successful resellers buy from Costco!