If you want to integrate Amazon with your Shiptheory account, you'll need to obtain your Amazon Seller ID and MWS Authorization Token. Part of the procedure is to click the Manage your Apps button.
Amazon has confirmed that this option is only available to those under Professional plan. If you want to clarify this, and get more information, please contact Amazon directly.
If you need assistance setting up your Amazon integration, or need anything else, please feel free to contact support.