I would like to have the Display name in the Email subject line rather than just the invoice number. Is this possible?

Hello merryfield@gmail,

Welcome to the Community! The subject line shown on the emailed invoice is based on the information set up in your Company settings. To check on the setup, here's how:

1. Click on the Settings icon.
2. Click on Account and Settings.
3. Click on the Sales tab, and select Messages.
4. Locate the Email subject line field and can update the information.
5. Click on Save, click Done.

Once done, try to open an invoice or create a dummy one and send it to your email to see how it looks. 

To add, since the subject line is based on the information entered on the settings and if the owner information keeps changing, you'll have to manually update the subject line prior to clicking save and sending invoice or sales form to customers as the information does not automatically change.

Post again in the Community if you have more questions about QuickBooks Online.