(Please ensure prior to that you are logged-in to your Amazon seller account as an admin user.)
1. Click on Amazon MCF Settings under Configuration in our app.
2. Click on "Add New Amazon Marketplace."
3. Add the new FBA marketplace you wish to add and click on connect. - If the US is in the existing FBA marketplace, add another Amazon marketplace. (This may be a different FBA marketplace in your case.) This will take you to the Amazon Seller Central page.
4. Once connected, it will enable the connection automatically between the Shopify store and the Amazon seller via our app for the newly added FBA marketplace. It will take you back/redirect you to our app again.