[ssewell]

Amazon Prime Now is already starting this (https://goo.gl/fSF4DR) in select markets. I used this last weekend and the delivery was perfect! You can opt to have an out-of-stock item either left off or replaced with a similar item. They will even send an SMS to request confirmation if they need to use a substitute product, giving you an opportunity to provide more feedback.

[dangrossman]

That's exactly how Instacart works as well. The shopper is able to call or text the customer to discuss substitutions.

[elithrar]

Correct - as well as choose from "best replacement", "replace with specific item", and "don't replace".

Since Instacart remembers these choices across orders, I've found this useful for making sure I get what I need.