I Mile Delivery refusing to supply source documents/fees for customs duties

I ordered clothing for my daughter from Shein, and then received a notice to pay I Mile delivery within 10 days. Their portal only allows for payment, no make up of how these fees are calculated is enclosed. I called and was told I need to call SARS for documentation and a subtle threat that if I don't pay the goods will be returned. SURELY SURELY I have a right as a consumer to know what I am paying for and to check the calculation? In this day and age of ***** and *******ion, I refuse to be bul**** into paying a bill unseen, what recourse do I have other than sitting on a sars call where I am higher than 200 on the queue - how can a Company be allowed to operate in this manner? Buffalo used to deal with logistics from China and they always supp**** the documentation.