Overcharged and Double Charged!

We rented out a car at Tempest King Shaka airport for two days. Having received an invoice today I noted that the price charged was not the proposed prince under the Kulula Vitality Scheme. Additionally we got charged to fuel and refuelling when we refilled the gas at the airport prior to dropping off the car. When I spoke to the consultant Sashnee Reddy she said I must send her proof of payment because it is their requirement to refuel the car and because I had used X kilometres in the car they assummed X amount of gas consumption. I find this incredibly bizarre because we did call out for a full tank refill and the petrol station was no more 500meters from the car rental place. This is shocking that because we are paying using our credit cards these service providers think they can dip their hands in our cookie jars as they please! I am just wondering how many customers rent out cars only to fall victims to these schemes.