Well, strictly speaking, if he has already paid then there is no "invoice" to be generated (since an invoice is a request for money which he has already paid)
HOWEVER if you go to your list of sold items, there is a drop down on the left from which you can select "Print packing slip and more". Click that and a box will pop up offering you the opportunity to save an "order receipt" to a PDF file which you can then email to him.
It would also seem that from his end he could just as easily bring up the order and produce a receipt for himself.
"Laissez-faire capitalism (AKA The Great Material Continuum) is the only social system based on the recognition of individual rights and, therefore, the only system that bans force from social relationships." ~ Ayn Rand
