Hi KR62
Thanks for your message. I could not get this to replicate in my own software. Could you try to clear the cache and try again. Could you also try with a different email address as well?
If you are still unable to add an extra email address to customer cards with a balance owing, could you please send me screenshots of the error message, a customercard with balance owing and one where there is no balance owing, adding that same email address.
As email addresses are private information, you can send the screenshots to me via Private Message (you can take a screenshot with a snipping tool such as Windows-key + Shift + S in Windows 10).