I rented a car for 3 days last October but went on to extend for an extra day, day before i was meant to return the vehicle, an amount of 1,536.68 was debited from my account for the extra day. I called in several times demanding a refund but with no luck

I rented a car for 3 days last October but went on to extend for an extra day, day before i was meant to return the vehicle, an amount of 1,536.68 was debited from my account for the extra day. I called in several times demanding a refund but with no luck because I was told they couldn't work out a daily rate since my booking was combined with my airplane booking which came as a combo.

I don't mind paying my way but no ways I'm paying that amount for one day car rental.