In the Payment section of the app, it is possible to toggle the Balance Auto Update on and off. The toggle is purple when it is on and white when it is off.
The selected Balance Auto Update amount will load onto the account as credit balance that will apply to all your rides. That amount will automatically reload the balance from the default payment method whenever the current balance reaches $0.
If you wish to prevent the next reload, simply toggle Balance Auto Update off before your ride. You will need to enable Balance Auto Update to ride again once your balance reaches $0. When you reenable, you will be able to reselect your Balance Auto Update amount.
To change your Balance Auto Update amount you simply need to toggle the switch off and then on again. You may need to have a $0 balance when you switch the toggle for the selection screen to appear.