The concept of "tipping" apparently does not exist in the Chinese culture. The delivery person is paid an agreed amount when an agreed number of deliveries are completed. Before the existence of mobile phone apps, a few customers may "round off" the order and let the delivery person to keep the difference but usually it is only about a dollar or two. This has become even more difficult now since transactions are made through the app.
On the bright side, most restaurants provide the motorcycles and cover the cost of gasoline.