2 hours late and a full day's rental charged!

Recently hired a car from Thrifty and drove it from Jhb to Dbn. Aircon did not work, so they changed the vehicle for us in the Dbn branch. We suppose to return the vehicle on Saturday evening but we extended an extra day and they billed us for it and now the new drop off day was Sunday, which we did but it was 2 hours late and then today they debit us for a full day's rental.