The option to choose a template to send a customer's form is unavailable, Kirbyconrad.

You can only customize your default email message in each customer form (E.g., Invoice, Estimate, and Sales Receipt) and use it as a standard/default message.

To do it:

1. Click Gear, and then click Account and Settings.
2. Click the Sales tab.
3. Click the Pencil icon in the Messages section.
4. Select a form under Sales form drop-down and edit/create a message for your customer. 
5. Click Save, and then click Done.

For more information, please check out this article: How to customize invoice emails.

Please let me know if there's anything else you need. I'll be around to help.