Excessive TAX and duties

I have ordered a parcel from the USA which cost me $100. I have however received an invoice of R1523.30. I cannot understand how I can be charged $100 for a parcel that cost $100. I have then searched reviews on the internet and saw that it is a common occurrence for Fedex to charge excessive amounts. The only way I can get close to the customs value is if I add the shipping fee paid to Fedex.

Could someone please advise how the customs amount came to R3035.00? I have no problem paying for customs and VAT and the administration fee for the correct customs value, but the amount is incorrect.