DHL charging me R1500 for mistake made by supplier

My supplier undervalued the goods that I bought from them online. The DHL guys asked me for invoice, they asked me for proof of payment, and I provided all the documentation. I know that goods need to have their import duties and vat paid for. I have no problem paying for this as I have done this before. However, this time around, because the supplier undervalued the goods, DHL and customs want ME to pay R1500 for a dress that cost R500. So in total, the dress that cost me R500 to buy should now cost me R3000. Who in their right mind would want to buy a dress for that much? I should not be made to pay for the mistakes of other people. I provided my information, so they should resolve this and make me pay what I owe, which is the customs duty and the vat.