I have (watched myself, that is).
I pick up the envelope in my left hand and turn it over to slit it open with my paper-knife.
It’s what everyone does (once they realize that it’s mail of interest, perhaps because of the postage stamp).
Printing a slogan on the back of an envelope is trivial – you are merely printing a pre-established file – and can be achieved with a shortcut keystroke or with the click of a mouse. If you are more confident than I you can tie the entire operation into the “PrintEnevlopeFaceSide” macro)
Now your client gets another message from you, besides the stamp, besides the contents of the envelope.