I've noticed higher costs too, but wondered if it had to do with having to order items separately instead of as a meal. For example, ordering a sandwich, fries, and drink, instead of a combo with the same thing.
When I found it, it was for a place called Ike's in the Bay Area. All of the individual sandwich prices were $1-$2 more than if you ordered directly from the Ike's app. It wasn't because of a combo price reduction or anything. (And funnily enough, Ike's uses DoorDash's white label delivery service to actually deliver the food.)