I know how important it is to make sure your taxes are being calculated correctly on your invoices. I can appreciate that this is something that you've worked with our team on and that you haven't yet been able to resolve things. I'll chime in and offer some insight into the best way to go about sorting this out.
If you're using QuickBooks Self-Employed, this issue will have to be handled by their designated support team. It sounds as though you were already in touch with them via email, however it's essential for their team to help you with this as they have access to all of your account-specific information.
If you'd like to chat with an agent in-product, you can do so by following the steps listed here: Contact QuickBooks Self-Employed Support.
In the meantime, perhaps other Community members with QuickBooks Self-Employed can chime in with their experience.