When printing a P.O. after it prints it asks if we want to email the PO.
 
We would like the same question asked after printing an invoice or perhaps this already exists and dummy me does not know how to turn it on.

You need to go to SD-Q, System Manager, and set the Multiple Print Dialog Box to A.  Then after you print or preview an invoice (or any other RTM) it will ask if you are finished printing.  If you say N, you can go back to the print dialog box and choose email.