Im not able to replicate this on my dev system or the www7 test portal but there are a lot different combinations of settings for tax, shipping, etc and I may not have configured my cart the exact way you have. Can you login to our test site http://www7.smith-consulting.com/ and try to replicate the error. This will determine if the issue is with the latest version of the cart or an issue with your specific dnn portal/cart install.
Please send an email to support@smith-consulting.com if you need a login.
Or you can wait until tomorrow and download cart v3.97 from the downloads page to see if that fixes the issue fro you.
-Scott