I suggest you don't contact Shenzhen. A set of certificates makes you miserable.

Just get the goods directly from foreigners. Factory and wholesale are cooperative relations.

The general wholesaler will say that there is a factory, but there is no factory. They have their own channels for getting goods, processing and selling.

I have been engaged in diamond wholesale for several years. I really suggest that you don't take the goods from the Shenzhen factory. Although most of them are reliable, there are also many liars.