I have tried that.  Checking "Show progress on line items (email only)" does not seem to change the layout of the invoice in any way.  The detail does not show when creating an invoice in QB.  Try it out.  Create a progress invoice and send it to a client.  There is a PDF attachment on the email which does not show any of the progress info.  There is a link to "Print or Save".  That leads you to a QB page where you can "view invoice"  That button displays the same info as the PDF that was attached to the email.  Where does the magic happen?  I have attached an example created in QB Desktop and QB Online.  Both invoices are for 40% of the same estimate.