We've been using Instacart for a while, so I can shed some light on fees, etc. For us, they cover Shaws, Demoulas, BJs, Petco and CVS. We've used all but Petco and CVS.
You can either sign up monthly or annually for their service ($9.95 / month) or pay a delivery charge. If you sign up for the service, the waive the normal delivery fee and give you prioritized service.
You pick from a catalog for the specific store. They list everything the store has and they do charge an uplift for each item. So, if you'd normally pay $1.00 at the store, you may pay $1.10, it varies. We usually get 90% of what we order, the rest are substitutes or not in stock. You can text with the shopper when they're in the store and make any changes, approve substitutions, etc.
Overall, we've be very happy, usually get our order same day or next day. I'd say the increased cost, including the tip is around 25-30%, but since we're not in the store impulse buying
, seems to come out about the same.