I had a similar problem when I sold a Maria+Popovi Martinez ceramic bowl to an art gallery in Texas. I just described the item as a Maria+Popovi Martinez earthenware bowl. The piece was then held at US customs. Apparently my description was not good enough. To me it was an earthenware bowl which was rather naive of me. US customs wanted to know more about this item before it could be released {sounded crazy to me} like what it was made of
{earthenware clay} So I emailed the gallery who very helpfully contacted US customs and explained that the item was Native American art pottery and not made of any dangerous substances and that did the trick, the item was released undamaged and I got my money from the gallery. I suspect you might be having a similar problem. The materials that make up the item might need better description and perhaps more focus on the art nature of the item as well as antique and collectible. A horrible situation to be in and completely unnecessary. Customs can be rather a mystery and frightening when trading the future of casual personal trade does not look good. I can only suggest no longer purchasing from outside your own country. I hope you can get it sorted.