If he is using DHL, there is not going to be an easy way to avoiding custom duty. Here is the reason why:
DHL, Fedex, TNT and all global parcel companies have custom clearance agents in every airport, and their job is to clear the duties on the spot for every parcel being received. The only time they cannot clear a shipment is when, the custom needs some documentation from shipper/receiver. As long as there is no documentation required, the agents will clear the parcel immediately, and then charge the customer for the duties. This is why they do not accept parcels with incomplete/missing documentation. Remember, their job is to deliver a parcel in shortest possible time, not necessarily cheapest.
I know this detail, as my father has worked in air/sea/land cargo business almost 40 years. So as per my understanding, if you use FedEx or DHL, and the Pak Customs open your parcel and put import taxes, you cannot avoid these duties.
The benefit of using a reputed courier company is that they will always be professional, on time and you always have a trace of the item. In services like Airmail Economy, most or all of these services are missing.
