Incorrect clearance codes leading to incorrect duties

We've bring in multiple shipments from overseas monthly through DHL. This week, they cleared shipments from two of our regular suppliers and charged us additional excise duties which we've never been charged before. It seems the used the incorrect codes during clearance (they were slightly different than the correct ones they usually use). Unfortunately, noone at DHL is responding to our requests to fix the issue and it's costing us a significant amount of money as a small business. We've always believed DHL was one of the few good courier companies still operating but this experience has been beyond disappointing and we'll have to speak to our suppliers about using someone else.