Hello,
When you are redirected from your custom gateway, is this error appearing on the ConfirmStatus page? Looks like it may have something to do with the tax calculation, can you tell me how your Shipping and Handing and Also Tax Setup settings are configured? Unfortunately I will not be able to your custom gateway on our test portal to reproduce this issue. Can you login to our test site 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 kevin@smith-consulting.com if you need login credentials.
Thanks,