Ideally this should not happen. Even with multiple items, the rate should show. Would it be possible for you to raise this issue in the wordpress support forum. This way, all customers who are facing this issue will get to understand the resolution of this.
Also, once you have posted this in wordpress forum, you can also attach the Request and response info in the thread so that our team can check the problem and help you. Here is a video that will help you to understand how to get the request and response by enabling the debug mode.