How do i get the invoice number to show up in the subject line when i email invoices?

Thanks for posting in the Community, @cboe. 

Let's go to your custom template and edit it so the invoice number will show up in the subject line when emailing invoices through the program. Then, make sure that there's an <NUM> in the Subject field. Let me guide you with the steps. 

1. Go to Edit from the top menu. 
2. Select Preferences.
3. Choose Send forms from the left menu.
4. Go to the Company Preferences tab.
5. Select your custom template. 
6. Click Edit. 
7. In the Subject field, make sure that there's a <NUM> beside Invoice. 
8. Click Save. 
9. Choose OK to close the Preferences window. 

The screenshot below shows you the third to eight steps.

Customizing email templates helps you personalize your company's brand. For more details, go through this article: Email Templates.

If your emails aren't using a template you created, there are two solutions on how to resolve them. Just either copy and paste the correct message before sending it or reset your Email Later settings. For more information, check out this article: Fix Custom Email Template Issues. 

I'm always here to help if you need further assistance. Have a great day, @cboe.