Feel completely **********

Why is it that the recalculator on http://ratecalculator.postnet.co.za/dom_search/ is incorrect? I sent a parcel this morning as postnet to postnet counter but was shocked with the exorbitant price I had to pay, some details, the package was 10.3kg with a volume of 50l x 40w x 27h. According to the rate calculator, this should cost R219, why in the world was I charged R425, that is almost double what I feel was still pricey but fair. This is misleading and I need th is looked into and to be rectified. This is not a next day parcel as I didn't request this and was also advised 3 to 4 days for delivery as well which is what I wanted