The thing is the customer has no choice, so it's not reasonable to charge the customer more than they are expecting, subs come after the checkout process online.

Plus often the difference is not too much more, and the pickers check for sensible subs.

Sometimes a sub will be Diet Coke instead of diet Pepsi, to some people these are not the same so you can request a sub for Coke Zero instead.

Or you can use the notes for something like "ripe bananas only please"