Hidden information when booking

Made a booking online, got to the collection after arriving in PE Airport and suddenly there was an issue with our booking. It was stated that my girlfriend is under 23 years old and now an additional charge will be added. Why was this not added with our primary booking? Only upon arrival to collect the vehicle, we were told that another R960 will be needed to get the car,while the original amount is only R711. This is ridiculous,if I knew about this beforehand,then I would have used another company instead. Definitely the first car rental and the last rental with this company!