Hi there, Kyle_KauffmanCPA.
Currently, QuickBooks doesn't let you include mapped or custom fields to the email templates.
When customers receive the email, all invoice details will appear when they click the attachments.
I'll provide screenshot for additional information.
Please check this article on how can change the basic design, layout, and info that appears on your forms: Customize your invoices, estimates, and sales receipts in QuickBooks Online.
Please know that you're always welcome to post if you have any other concerns. Wishing you and your business continued success.