Hi Matt,
I have tried to re-produce this issue on my test portal and was unable to recreate the behavior you reported. This may be because this issue has already been addressed in a recent release as I am testing on v.5.0, or I may not have the exact same configuration as you. I would like to have you login to one of our public test portals and configure the cart the way you have your cart configured and try to replicate the issue. This will determine if the issue you are reporting 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 for a link/login to the test portal.
Thanks,