I too would like to strongly suggest that Quickbooks Online add more variables besides just [Invoice No.] to be used in the invoice email templates. I don't know why this issue is marked as "solved" when there definitely is not a solution to the original posters request. It is a shame that I cannot insert things like [PO Number] or [Tracking No.] to this email template and must have our bookkeepers and sales administrators manually insert these elements with copy and paste every time. A bit more flexibility would be nice. (and please no more comments from the Xero people suggesting we switch platforms). I need this to work in Quickbooks Online. 

Thanks!

Brad Bennett

Pacific Pickle Works