Unexplained charges

I recently received my SAD500 for the import charges on a shipment. My previous 2 import came to 43% of the total of the invoice - which is correct on the standards and percentages that SARS prescribe. But my 3rd import came to 59% of the total of the invoice and DHL can not explain why. On there they have 2 x calculations for import vat / duties (which they fail to explain what it is for) These are exactly the same products, from the same country & the same supplier - so these are constants. I have asked them NUMEROUS times to please just explain & show me the calculations of how they came to the amount that I must pay, but I'm blatantly being ignored. My concern with this is that these are not for personal & I'm running a business off these imports. I can not import items with inconsistent import vat / duty fees from the courier.

I have emailed them (after they have corresponded with me via email) and asked for an explanation but still being ignored. The lack of correspondence on this matter is questionable. Surely I'm not wrong to expect them to explain what I'm paying for - if it doesn't add up? shipment arrived 31.01.2017 & I still don't have my stock due to me refusing to pay for something they don't want to explain.