In Canada at least, the Real Canadian Superstore chain has set up InstaCart as their "official grocery delivery" partner. When you go to the Superstore website and click "Shop Delivery", you go through an InstaCart flow.

Although I suspect complaining to either the store itself or to InstaCart is going to be relatively futile, it seems that having the store managers bubble their complaints upward to regional managers may be more effective than complaining directly to InstaCart. Corporate makes decisions like that partnership, and if all of the store managers in a region are making the same complaint to HQ, maybe something might change. Maybe.