Overbilling customers

The Courier Guy's SWAD system is overbilling their clients. We recently shipped a parcel overnight from Bloemfontein to Randburg. It was a little box, no bigger than 20 x 15 x 5cm, and was shipped in a flyer. It's weight was 0.12kg. The online quote for the shipment was R115.00. When we got the invoice, we were billed for a staggering R2081.50!!! When I inspected the waybill, I saw that they have captured the parcel dimensions as 378.1cm x 75cm x 7cm! I'm trying to correct this with their accounts department, but their support system makes one feel completely helpless. I'm also wondering how many of our other shipments were overbilled, and how many million shipments for their other customers were overbilled. One kind of trusts that their system is accurate, but I now see that it can not be trusted at all! You have to check every invoice for correctness of billing. And good luck to you if you've received a zendesk support ticket... I don't think there are actually anyone working in The Courier Guy's support department - it's just an automatic ticketing system.