We tried Instacart in August. We always get groceries, always rent a car for 24 hours and purchase at Publix. It's a bit time-consuming, but it works well for us. Well we decided we would pay the extra and have someone else do it and avoid the inconvenience of picking up and dropping of a rental.
It did not work for us. The girl that shopped for us was very nice and was doing her best but we were doing a very large order to be delivered at 8pm. Many items were out of stock so there was constant texting back and forth while she did our shopping. We had meals planned for 6 people for a week so the plans were being thrown off on the fly. Example, no lettuce meant no lettuce for salad for dinner or sandwiches for lunch. So if we don't have lettuce, we don't need as many tomatoes, onions, peppers etc. We also had soy and dairy allergies to deal with and didn't know the American brands as well so it was hard to substitute quickly. The order was late, again didn't blame the shopper. She started texting just after 6pm and finally delivered around 930pm, we skipped bell services and loaded the groceries to our room ourselves. We were racing to get done before the fireworks at 10pm. My daughter who was the one texting felt like she couldn't do anything until the order came. In her words "never again".
For the original poster, a lot of our experience will likely not apply to you, but I'm not sure you'll save money using this method. The prices on Instacart are inflated (we were clear about that ahead of time, it's on their site) and then tipping shopper and driver. In our case we were using them so that we wouldn't lose half a day of vacation getting groceries, in the end we lost half a day anyway.
