They're warning about the reaction of customers who encounter higher prices on the items they're looking at. Even if we assume/pretend that the total for some

They're warning about the reaction of customers who encounter higher prices on the items they're looking at. Even if we assume/pretend that the total for some specific order might not change, they're argument is that higher nominal prices on the menu will discourage customers from ordering as much in the first place and thereby depress the industry. But of course, even if so, that's by way of customers getting clearer insight so it's not the most convincing argument from the perspectives of ethics or even market efficiency.