I was wondering the exact same thing. I'd imagine that they'd use shoppers who have personal memberships and then reimburse. If they had Instacart-only memberships, it would be much easier for Costco to target and cancel those accounts.

Any agreement between Costco and Instacart would basically be endorsing a method of skirting the annual membership fees, something Costco would most likely never be interested in. However, as a Costco member, I would absolutely love this. No more "Cart" rage on Saturdays!