Is it possible to add the customer's Display Name to the title of the invoice email template? I can't seem to find any mention of this as a possibility. How do I do this?

Hi EdiwinH-EarthCare, 

Thank you for reaching out on this and welcome to our community. 

I ran a few different ways in my test file and I can confirm said function is actually not supported in QuickBooks Online. I would like to encourage you to submit this as a feature request in our feedback session. Here is how:

How do I submit feedback?
 

Thank you and please let us know if you require any additional assistance.