Instacart nominally does this, and then the shopper went ahead and gave me bad substitutions anyway. I mentioned in another comment here that you're getting real-time notifications about substitutes while they're shopping (there's no real-time inventory available, so they discover an out-of-stock item when they go to grab it). If you don't answer their substitution question quickly enough, they'll just make a decision (skip, pick the sub you asked for, pick a random similar item).