Unsatisfactory customer service from AVIS

On 21Feb2018 I drove 40Km however was charged for use of 400km. I have send an e-mail disputing this, however no response received. Thus I contact the consumer helpline by telephone. As I hang on waiting for the call to be answered, due to their lines very busy, a message playing asking to send any queries by e-mail and a promise to receive response within 24hours. After a while the phone was answered. I firstly asked why I do not receive an e-mail response. Ellezi Kiewits said that they experience a huge back log on the e-mails. I then explained the reason for my call, for which I could not get a reference number for, and secondly she promised that I will receive feedback within 24 hours. Now, 5 days later, still no response. I wonder whether Avis service is so bad that they have a backlog of 2 months on the mail complaints?