Thoughtful responses. I am looking at this from a very UK perspective, so much of what I've said maybe (and indeed sounds like it is) not relevant to the US. Over here UK supermarkets have matured their online businesses very well. It's no more expensive than in-store and delivery is free to cheap depending on how narrow a delievry window you can handle. Plus of course the US is much larger geographically and much more car-focused, so it might be a much more viable business model.

Either way - it's a big project. Good luck!