Valuable objects, i.e. gold, white gold, silver, gemstone, jewelry, gem, coin, banknote, bill of exchange or any other printed matter which can execute the payment to the holder, traveler’s check, or any other valuable objects are prohibited for the international express courier service (EMS).
The mentioned items can be delivered by international express courier service with insurance service (add-on). The conditions will conform to the policies of the destination country. (Ref: https://file.thailandpost.com/upload/content/_5d5cc7b79b3e1.pdf)