Lightroom Classic: How do I print inside a store bought template/invitation

You could create a custom print template in the Print Module that matches your card size then make a custom Identity Plate for the words.

I found these free greeting card templates with a quick Google search:

http://www.redrivercatalog.com/cardshop/help/lightroom-greeting-note-card-printing-templates.html

Try a Google search for "Lightroom Greeting Cards." You will find a ton of ideas.