Where do I add the sender contact details and address that’s shown on the shipping labels?

The sender address is automatically pulled from the “Shipping” section in your FlavorCloud account settings. When you set up your FlavorCloud account, the shipping location defaults to the account address. 

If you want to change the origin shipping location, you have two options: You either can import warehouse location(s) from Shopify by clicking on the “Location” section, or you can enter your warehouse location by selecting  “add address” (in app 2.0).