Driver tip and review system is unclear and needs to be reviewed.

I always include a tip for the driver. Then the app asks to review the drivers service and includes the tip amount. BUT nowhere does it state that when I accept the review that the amount (which can be changed) will not be ADDED to the tip I have already given. So, I set that amount to zero and the tip is refunded to me??????? Now I can't re-tip the driver???? This is a stupid system that's not intuitive or explains clearly. That driver now thinks I'm a doos for retracting my tip???