Welcome, jolie.lotz@ybs.u. 

It's great to see a new face here in the Community. Let me share some insight on the subject line within the invoice email. 

In order to save the subject line successfully, you'll need to at least have the work "invoice" included in the name. (Example: "Invoice Jolie," or "Jolie invoice.") 

Here's a guide with more details on invoice templates.

If this doesn't benefit your business, I recommend submitting feedback to our Product Developers so they can consider this in QuickBooks Online in the near future. 

Check out our Blog site to keep up with what's new in the system. 

I hope this helps answer your question. If you have any other concerns, don't hesitate to come back. Wishing you and your business continued success!