Presumably you have created report with:
- a group header for the InvoiceID (invoice number, date, and address
panel),
- a detail section (for the line items on the invoice),
- a group footer for the InvoiceID (for the total.)
In the Sorting And Grouping box, add InvoiceID as second time.
For this 2nd one, set Group Footer to Yes.
This gives you 2 group footers for each invoice.
You can now place the text you want in the new group footer.
By setting the Force New Page property of the sections, this will end up on
the back of the invoice.
The only remaining problem would be what happens if you have a really long
invoice with lots of items so it spills over onto a second page. That would
throw the double-siding for all the remaining invoices in a print run. If
you are concerned about that, see:
Duplex reports: start each group on an odd page
at:
http://allenbrowne.com/ReportDuplex.html