Overcharging

I hired a car from Avis international Airport on 21-24 September 2018. On my way to Eastern Cape I had tyre burst. I phoned Avis and they told me I can go to the nearest Avis to get the spare wheel. I went to Avis East London, initially they did not want to give me the spare wheel, after aguing with them they say i terms of Avis policy they will give me another, now Avis Cape Town is charging me for not returning the same car to Avis international Airport. The invoice is double the quotation I received when I hired the car. Please help on this unfair bill.