The rate lookup API version that is used in eCart is v3. As far as I know there is no testing server option for this version of the API. In order to interact with the API you will need the production URL and you will need to have this option enabled for you by USPS. To do this you will need to contact USPS and let them know that you are working with the v3 API and need the production server URL for this as well as access to the server.