Avis DON'T know what they doing

Had a 60 day contract with Avis Cape Town. Long story and many calls we find out they opened 2 contracts. Eventually they "rectified," the matter. One week before contract expires they reverse the entire 15 000 back to my credit card. They then took another amount. They have messed up credit card.To add insult to injury they now claim I owe them one payment. Avis staff don't know what they doing and don't want to take responsibility for their actions. The NCR has rules. Avis go and find out what they are. Messing up my credit card and then harassing me! Really,?