Overcharged n no feedback from them

I have no idea how they work out they invoices. However I do know that they just randomly send u any invoice amount that suits them. Last month my Aunt got the exact same thing from Dubai with a value of 300AED she was charged R1088 for custom duties. This month the exact same thing I got in n they charged me R3088 custom duties n taxes. How on earth they calculate I ve no idea Top it all I ve emailed them numerous times I recieved no response from them. Its been 2 wks now .No update. Totql *******.