Just to tag onto this, you should use the option to add the tip after the delivery. If you don't then the app will just subsidize themselves with your tip.
For example each delivery is an offer to the delivery driver so a delivery will be 10 dollars. Well if you tipped 8 dollars on that order then the service will pay you 2 dollars and the 8 dollar tip will be the rest. But if you start by tipping zero then the service will pay ten dollars for the delivery and then if you add the tip the gig worker will get 18 dollars for the order.