Avis system is failing

Avis frustrated me,I rented a car with then for 2weeks and a week after I returned the car Avis deducted money from my account which left me with a minus balance and I accumulated extra charges.When I phone my bank they told me that Avis haven’t instructed them to release reserved funds that was reserved during the time I rented.Emails have been seen sent and several calls but nothing has been done.I mean why can’t Avis instruct my bank to release funds before taking my money from my account especially because they already reserved for all transactions made while renting the car.Who is suppose to cover insufficient funds charges.