I mean they hide it as best they can. Big restaurants like Applebee's you'll see "2 for $28" not priced at $28 so you can guesstimate the squeeze but otherwise you kinda have to go straight to Starbucks or McDonalds using a mobile app to order your "usual" to compare "here's what it looks like if I use DoorDash, here's what it looks like if I go myself," to find that the actual delivery fee is some $20-25 per order. Even worse, I'm pretty sure that they test algorithms to try to selectively lower this for new customers so that in the early days when you're more aware of the cost it seems like a steal.
Of course, you can arrive at the $20 just by thinking, "okay, I need someone to go do an errand for me, they'll have to drive to the restaurant, wait there for 15-20 minutes, and then bring it back... so it'll cost $15 for the hour of their time plus a few bucks of overhead for the platform plus a few bucks of messed-up-my-order insurance..."
Which gets us to 5 years from now when the DoorDash killer comes out, it'll be called Kourier or something starting with a K, and it'll start with trying to give Target a way to call up some extra trained Target employees, but they're cross-trained in packaging orders for K. One person will pick up 10 carefully-packaged K-orders, take them all to the central delivery hub, they'll get sorted into driverless cars that plot through some neighborhood some 10 stops, it'll be marketed as a real Amazon-killer and fly under DoorDash's nose -- InstaCart might balk, but DoorDash won't. Until they reveal some pizza-delivery partnership and suddenly within a year every restaurant has some K-employee working for them, whose job it is to batch orders down to the bikes that come by.
Sure, delivery times for Kourier will be 75, 80 minutes long at first. People won't mind because you pay $4 for delivery instead of $20. And Doordash/Amazon won't die, Amazon will just buy Kourier and DoorDash will focus on more rural locales.