Thanks for joining this thread, StephMPC.
 

I can certainly understand how a feature for choosing a custom template when using your Combine forms to a recipient in one email option could be useful and have submitted a suggestion about it as of today.
 

You can also submit your own feature requests while using QuickBooks.
 

Here's how:

1. In your top menu bar, go to Help, Send Feedback Online, then Product Suggestion....
   
2. Choose a Type of Feedback and Product Area (optional) from the available drop-down menus.
   
3. Enter any suggestions/feedback in your Here is my suggestion: field.
   
4. If you'd like, include your name/email in the My name is: (optional) and My e-mail address is: (optional) fields.
   
5. Select Send Feedback.
   

Your feedback's definitely valuable to Intuit. It will be reviewed by our Product Development team and considered in future updates. You'll be able to find your product's latest news by checking out the QuickBooks Blog.
 

As a workaround, when combining forms in one email, you can manually compose the BODY section of your communication. This way, customers will see the text you want displayed.

I've additionally included a detailed resource about emailing sales forms that may come in handy moving forward: Email sales forms
 

Please don't hesitate to send a reply if there's any questions. Have a fantastic day!