Issac should have known that when he encouraged Aldric to have other hobbies that excluded killing others, he would end up being his errand Fae. He underestimated the dark prince's passion for his work and now has to perform the menial task because Prince Aldric was paranoid.
Some of the items on the lists were fragile and required extreme care, and only Issac understood what they meant to Aldric - and he could easily punish him if he failed. But that was not the end because some of the goods were banned too but Issac had the network to purchase them in secret. That is to say, Issac was the perfect Fae for the work.
He left the castle and was on his way to meet with one of his contacts he trusted and had arranged a meeting with. He exchanged pleasantries with some of the Faeries on his path and walked for a while when he had this eerie feeling of being watched.