This is essentially why I don't order grocery store delivery via the internet. I tried it a couple times, and when the item I wanted was out of stock, they reasonably wouldn't pick a substitute. I think there were some options to specify a substitute, but it's just too much work to make a list and then make an alternative list for every item that isn't there. So I end up going myself and making those decisions ad hoc, which the store picker can't/won't.