Can I deal exclusively with a single supplier on Shopify and does it calculate import tax?

[Steviepeeps]

Sorry, I’m really new to this so I don’t know how silly my questions might sound…

I have a potential supplier in China that I’ve been in contact with for a couple of months. I want to send up an international website and also trade on eBay etc. With the plan for me to sell (based in UK) around the world with shipping sent directly from China.

if he joins as a supplier, can I deal exclusively with him on shopify? Does shopify automatically calculate the import tax to each country that goods are sent. 
Mia this the place to do such a thing or is there an alternative?