Hi there, @ewright12.

QuickBooks Desktop lets you set up multiple email templates as required by your clients. I'm glad to assist you on how to add your email template in the invoice or estimates. 

Here's how:

1. Go to the Edit menu, choose Preferences.
2. Select Company Preferences, then click Send Forms.
3. In the Delivery Default drop-down arrow, pick E-mail.
4. From the Email Templates, choose either Invoices or Estimates.
5. Hit Add Template then enter the email address.
6. You can select Insert Field to put a dynamic data field in your email.
7. Click Save, then press OK. 

Once you're done adding an email, let's create a sample invoice or estimates to verify if the email will take effect. You can send an invoice/estimates with their email template by clicking the Email drop-down in the Send Forms menu. Let me show you how:

1. Click Customers, then choose to Create Estimates. 
2. Fill out the needed boxes and put a check-mark in the Email later.
3. Click Save and Close.

After that, you can go to the File menu and select Send forms. From there you can select one of your templates when you send a batch of emails.

I've attached a screenshot for your reference:

You may check out this article for additional information about the process: Create custom email templates in QuickBooks Desktop.

Also, feel free to read these handy articles for your reference: These articles will guide on how to fix email template issues, create custom Field, and customizing forms:

• Fix custom email template issues
• Use and customize form templates
• Create and use custom fields

Don't hesitate to leave a comment below if you have follow up questions. I'm always to help. Take good care.