There are several reasons that the consignee may have been charged duty and tax. When you book a shipment as DDP, new labels must be issued. If new labels were not issued or used, your consignee would be charged. If the labels were generated and used, the problem may be that the customs office charged your consignee incorrectly, or that your destination is a country that does not allow DDP shipments.
To ensure your consignee does not have to pay the duties and taxes, you can contact the local customs department and dispute the charge. State that all fees should be billed to the sending party, as the shipment was sent deliver duty paid.
If this fails, contact our customer services team who will help you rectify the situation.