When arranging the shipment by courier it often causes problems while making customs clearance and then delivering the goods to our warehouse.
Because for customs clearance we have to use DHL express (they have very long period for customs clearance release and we have to pay additional costs for storage during this period) and then we have to pass the goods to our forwarder what can cause the delays also.
That is why we would like to use standard air shipment.
Moreover, if you say that in terms of costs there is not much difference, then using courier service is useless for us and even will cost more.