Hello wakeupinmaui, 

Thanks for providing such detailed information. Let's work together and go through your concern one-at-a-time and resolve them. 

Are you referring to creating a new template using an existing one? If so, you can use the Duplicate or make a copy of a template functionality in QuickBooks Desktop. This allows you to apply the design and format of a customized template to a new one. 

Here's how:

1. Go to the Lists menu at the top menu bar, then Templates.
2. From the Templates list, select the template you want to copy.
3. In the Templates drop-down, choose Duplicate. 
4. Pick the type of form you want this template copy to be used for.
5. Click OK.

You can modify it depending on which information you want to add. If you want to use the new one to your invoice, just toggle the template you want to use . To do so:

1. Open an invoice.
2. Choose which template you want to select in the Template field (see screenshot 1).

Check out the second part of this article for more details:

Use and customize form templates

Now, for your second concern; you'll be able to add/alter the cover email and get rid of the due date detail in your transactions. This can help save your time since there isn't a need for you to manually remove it prior to sending the invoice. 

These steps can guide you through:

1. From the Edit menu, choose Preferences, then Send Forms.
2. In the Company Preferences, choose Email as Delivery Method Default.
3. Select the Template Type you want to create.
4. Highlight the template you want to modify, then pick Edit.
5. Remove the [Due Date] fields.
6. Click Save. 

To set the template as default, make sure to select Default prior to saving it. To know more about how this functionality works, check out the article below:

Customize email templates in QuickBooks

These steps should get you going!

Please note that we are committed to ensuring you can get the best help possible in using QuickBooks Desktop. If you have questions with the product, feel free to let me know. I'll be around.