HK act as gateway for goods import & export in between worldwide & China, the importation/exportation either by normal trade or courier etc. 

Different kind of goods have different kind of import & export regulation in China. e.g. Cosmetics via normal trade import to China require filing issued by authorized Foo & Drug department.

For Customer's goods need to be sold formally in China, or Customer need to sell their goods in China retail shop, their goods should be imported by normal trade. Then customer can have Customs declaration paper for selling.

For Customer's goods need to be sold via Ecommerce channel in China, then they can via HKG import to China by courier mode. The cost will be charged per kg inclusive China import tax.