Uber extra amounts added to rides

As of late (last 3 weeks) I have been charged a certain amount for rides upon request. Upon arriving at destination I'm always charged an extra R10 or R20, this happens almost every day wether pay via card or cash. On card they deduct the correct amount for specific trip, a few days later I will get a message I owe them for that trip. Not in any of these instances has the trips been delayed due to heavy traffic so I can not understand why an extra amount is always charged. Thus far all my trips are up to date and I don't have amounts outstanding may it be cash or card but yet I still get told after my trip is done I owe the driver and extra R10 or R20.