How do I create an invoice template that I can copy and use over and over just editing small bits of it?

There's no option to create a copy of your invoice, Jenny.

When you create a template in QuickBooks Online, it can be applied to all your invoices, since it's universal. If you want to add modifications from time to time just select the current template of your invoice. 

Here are the steps to customize your invoice template:

1.  Click on the Gear icon.
2.  Under Your Company, select Custom form styles.
3.  Click on Edit under Action, and choose the part of the invoice you want to customize (Header, Body, or Footer).
4.  Navigate to Design/Content/Emails, and check the box based on your preferences.

Once done, you can now use it, and enter the necessary changes.

To do that:

1.  
2. At the upper left menu, go to the +New.
3. Under Customer, choose Invoice.
4. Scroll down and click on Customize, then choose Edit Current.

Moreover, I've added this article to learn more how you can personalize your sales form: Customize invoices, estimates, and sales receipts in QuickBooks Online.

You can also check out how you can manage your transaction for future reference: Record invoice payments in QuickBooks Online.

If you have more questions related to invoice template in QuickBooks Online, you can always come back to this post and leave reply. Stay safe!