Courier Guy not taking care over overcharges collections, bill client and takes months to try resolve the issue

Been a business client for several years and recenlty received extremely bad service from The Courier Guy. Several attempts to rectify an over charge has come to nothing and my company has to keep paying in money to keep us in credit in order to make use of their services. They call the shots since they insist on a client being in credit to allow making use of their services. But when they owe a credit that is never taken into account. Their is also zero customer care unless you use online services. No verbal solutions are offered. We are a small business and can ill afford to keep ourselves in constant credit even if the Courier Guy has made an error and is taking several months to resolve the issue.