Great to have you here in Community, user85451!
I can help you edit the subject line of your email.
You'll need to manually edit it every time you'll send an invoice or an estimate. After clicking on Save and Send when you're about to send the email, you can edit the subject to edit the invoice number and add your customer's name.
Currently, the automated way is currently not available in QuickBooks Online.
We appreciate if you can send us feedback by clicking on the Gear icon and choose Feedback. This way we'll be aware of the things that we need to add and develop the product.
Keep me posted if there’s anything else you need, I’ll be around to help.
