If you follow the instructions below and use the "custom template"  the functionality is still not working correctly.  When you email the invoice to a customer and they click "Review and Pay"  then click "View invoice" they see the modern version of the invoice with the Product and Service.  They do not see the custom template that you want them to see.  This is a defect.  The system is not operating as it is expected to operate according to this post.