Welcome to the Community, @Ariel13.

Let me share with you some insights about customizing an invoice email. Currently, there isn't a setting that can automatically populate the Project name in the email subject. The only workaround they have in including the Subject name into the invoice email is to set or add it manually in the Account and Settings or in the template itself. 

Here's how:

1. Go to the Gear icon.
2. Select on Custom Form Styles.
3. Click on your invoice template.
4. Go to the Emails section.
5. Click the Subject Tab.
6. Enter the Project name + word (invoice)
7. Once you have finished, click Done.

Furthermore, I'm adding these helpful articles to guide you in customizing your invoices and recording your invoice payments: 

Customize invoices, estimates, and sales receipts in QuickBooks Online

Record invoice payments in QuickBooks Online

Let me know if you have additional questions about invoices and other QuickBooks-related concerns. I'll be here to lend a helping hand.

I tried creating different templates, each with a different project name in the subject line, but it is only letting me choose one option for all templates. Is there a workaround for that?

Hi there, @Ariel13.

I understand that you are looking for other ways to automatically include the project name in the email subject when sending an invoice. Unfortunately, there is no other workaround setting available in QuickBooks Online at the moment. However, you can manually add the subject name in the Account and Settings or in the invoice template itself.

I recommend that you send feedback to our Product Development Team regarding this issue and share your suggestions with them.  I will also forward your feedback to them, so they have a chance to look at the issue and fix it for you.

To send your feedback directly to the team, you can follow these steps: 

1.  Go to the Gear icon
2.  Select Feedback. 

In addition, here are some helpful articles to guide you with customizing your invoices and recording your invoice payments in QuickBooks Online:
 

• Customize invoices, estimates, and sales receipts in QuickBooks Online
• Record invoice payments in QuickBooks Online

If you have any further questions or concerns related to invoices or any other QuickBooks-related issues, don't hesitate to contact us. I'll be happy to help.