Hey there, @wakeupinmaui.
Welcome, and thank you for joining this discussion here in the Community. Also, thank you for providing me with what you've tried so far to resolve this. Since you've tried everything recommended prior, I'd like you to try one more thing to rule some probable causes. Do you mind changing your form template to a different one in QuickBooks? The reason for this is that you may have a broken template, which could very easily be linked to why the service data is showing instead of the due date, and why the "due date" is showing in the first place.
For your reference, I've included a link to an article on using and customizing form templates, just in case you need it down the road: Use and customize form templates.
After switching to a different template, try sending a mock email of the same invoice to yourself and see if the cover letter is showing in the same way. Be sure to comment back with your findings so I can assist you further. Should you have additional questions about this or anything else QuickBooks, don't hesitate to ask me here in the Community. I'll keep an eye out for your response.