Hi @topnotch719
As our valuable QuickBooks Online User, we always wanted to you to experience seamless transactions for you. And emailing you Invoice is one of our products for you to use run your business.
Allow me to provide the steps for you to change the Invoice Email Subject Line
1.) Proceed to Gear Icon.
2.) Select Account and Settings.
3.) Look for Company.
4.) Complete the information you need to fill up.
5.) Once done, you can go ahead to general Info and fine business name, click Edit.
6.) Under company name, there will be shown on sales forms and purchase orders connected to the Business Network.
After following these steps, please go ahead and check to your subject line if it was successfully changed.
consider scanning these articles for more guide:
Edit company settings in QuickBooks Online
Set up and send progress invoices in QuickBooks Online
Thank you for having time reaching us, if you have any other inquires about our Custom form styles. Please don't hesitate to leave us a comment, and I'll be delighted to assist you.