Car hire warning

I rented a car through discovery using tempest car hire, I was just invoiced 1/4 of the rental value additionally which I was not informed about because I had a code 10 license for a code 8 vehicle, so im being penalised for being over qualified, the fees I paid though discovery were not the only fees I had to pay, upon collection I had to pay costs and now this, this is utter bull**** and I advise people to stay away from using this company as they take advantage of customers knowing very well they have your deposit, unfortunately Discovery is using Tempest who is exploiting their customers