We have used Peapod a few times, a grocery delivery service here in the Northeast serviced by Stop and Shop. I couldn't figure out how they offered it so cheaply, as the prices are the same they are in stores. They charge delivery ($7) though you can often get offer codes.
Then I found out that apparently orders are serviced from a separate warehouse, where everything is categorized and optimized for online orders. It makes sense -- I couldn't picture guys walking through the store collecting 10 carts worth of orders.
Unfortunately even though it was only slightly more pricey, it still added to much to our household costs since groceries are such a huge part of a family's budget (avg family of 4 spends $770/mo). Not to mention delivery added on to the cost as well.