The tip suggestions are based on your settings to encourage better tipping practices. You can change the percentage amount to give clients several options during the checkout process.
If your client wants to add their own tip, they can tap the "$" button to type it in. That is always available so you don't need to set anything.
https://s3-us-west-2.amazonaws.com/secure.notion-static.com/f4234a06-63a3-42d5-a36d-d508994925dd/Simulator_Screen_Recording_-_iPhone_11_-_2021-05-12_at_21.55.28.mp4