Artist's materials coming into Turkey seem to be dutiable at one of the highest levels. Mrs KKOB, the artist in our family, used to bring her materials over in her luggage or via friends, and never imported via carriers or the postal service.
If the package is being held in Istanbul I don't think it'll be forwarded to Fethiye Customs until they've had payment of the duty from you or the carrier. Even if it's forwarded to Fethiye, don't expect them to make any effort to contact you. You'll have to go to them with as much detail as possible regarding consignor, consignee, carrier, consignment note number etc. If it was sent via the postal service you'll need to go to the PTT offices above the Otogar. If it was sent by a carrier, you'll have to go to their local office or agent. Either way you'll end-up at the Customs Offices by the harbour for a release note and then you'll have to go back to wherever the parcel's being held. Allow yourself at least a few hours to be to-ing and fro-ing and be prepared to pay "admin fees".
Find out from the carrier how much duty the Turkish Customs in Istanbul are asking you to pay and then weigh-up whether it's worth paying it, or writing the goods off.
