Excessive bill not declared upfront

I arranged for the delivery of a water pipe (weighing 2kg) from Pretoria and collected the parcel tyesterday from PostNet Tokai today. I was advised that it would cost R1300 for a same delivery and R185 for an overnight delivery. I opted for the overnight delivery option as the other option was too expensive. Today, I was advised that the courier cost would be R2200 based on the diamatric weight of the parcel. I queried the amount with the owner because at no stage was diametric weight and cost of R2200 ever discussed with me. I would not have agreed to courier the parcel at this price. The owner of Postnet Tokai explained that their pricing was based on the diametric weight of the parcel based on the height, width and length dimensions of the parcel as opposed to the physical weight. The owner insisted that the diametric weight and associated cost were discussed with me which I disputed. I agreed to pay R2000 as I needed the part to fix my car. I am very unhappy at the manner in which I was treated and would like to expose Postnet for not discussing their pricing structure with me, yet they insisted that I pay a fee which was not discussed/agreed with me.