This exact thing is happening to one of my favorite local pizza places. Very frustrating for them.
The smart thing to do is krep taking the business and just put a leaflet in the pizza box/whatever saying ordering directly is cheaper/faster, if that's the case.
Sometimes the restaurant doesn't know that something is being ordered via another channel and cannot add sufh a slip.
Also there are uncountable reports of them fucking up the delivery itself by not having appropriate containers, when the restaurant they are dealing doesn’t actually do deliveries. That will lose you customers and hurt your brand.
I think the smart thing to do is to trust that people who have spent years doing a thing have more insight into the topic than a random internet forum participant.