Both Delivery Duty Unpaid (DDU) and Delivered at Place (DAP) are Incoterms used to describe who is the importer of records for a shipment. DAP has replaced DDU in official usage, but they mean the same thing: you’re making your customer the importer of record.
Your customer is responsible for paying any import duties, customs fees or import VAT. Their order will be held at customs until they have paid. If they fail to pay on time, the goods will be returned to you. Once all the taxes and fees have been paid, their order is released from customs and continues to its destination.