E Invoicing requirements are becoming more of a legal requirement across multiple countries, the business requirement here is to transmit invoices electronically via XML instead of via email or printed matter, many of these businesses require that a copy of the invoice is embedded into the XML document. 

Standard processes allow for the generation of the posted invoice to be included as an attachment, but this will only work if the invoice printing process is initiated at the same time otherwise the posted invoice will not be added as an attachment.

The requirement is that the if the customer record has the "eInvoice attachment" flag set to YES, then a PDF copy of the invoice is generated at posting regardless of whether the invoice is printed or not.