Most grocery stores that offer their own delivery services do so for much cheaper than Instacart: they charge normal prices for items, and put all the delivery costs in the delivery fee where it belongs. For example, using Kroger's delivery service instead of InstaCart can be nearly 50% cheaper even before customer-card discounts because (a) Kroger charges the same price for items as you would pay in-store (and will offer sales prices if available), (b) Kroger generally charges a fixed fee for delivery, and (c) uses employees in locations where delivery is offered. The caveat is that most grocery stores have smaller delivery radiuses than Instacart.

Edit: Kroger owns a lot of chains; some of the chains might mark up prices on delivery items a bit, but the stores I have used do not.