Shopify would not be an exchange, you can't returns your coins for money. you can only buy coins from a shopify store. If you would like to sell coins for cash, you would have to setup your own store, provide shopify with your banking information and sell the coins that way. With this model you know who the buyer and sellers are. it's traditional commerce versus an exchange.
Ya, I got you 
So, your idea is to bring currencies (created to operate on a decentralized platform) into transactions on a centralized platform like a bank, huh? it doesn't have to be a bank, your merchant store could be a bitcoin store instead of shopify. you can sell stuff for bitcoins if you don't want to get the bank involved.