Overcharged

Sent a parcel off in a tube. Dimensions 120x8x8cm. Volumetric mass of 2Kg. The quote was R97.75 as usual for this size parcel which we often send. 2 months later we received an invoice and it was for R373.75! We were charged for a volumetric mass of 7KG! Ive been emailing and trying to sort this out for a month now but nobody can help me. We give this company so much money every month but as soon as there is a problem they refuse to help and we receive no feedback for weeks on end. Really thinking about changing our courier services.