Why the TNT not disclosing their fees upfront?

sadly enough this was my first time experience with the TNT. I had ordered the item worth USD 270 and asked to pay for TNT services USD 215. The item arrived to SA and they contacted me with email to pay additionally R 380 for their custom clearance fee. This excluded the custom duty fee. This fee was not disclosed to me while ordering the item. When I have asked why I was double billed the agent just ignore it. They also asked me to signed declaration that the SARS may fine me (!!) plus sky rocketing charge if I am late with my payment (!!). I was hold for a "ransom", pay it or loose it. I have informed the supplier of this concern.