When I have to send backup documents to a recipient, I email the invoice to myself. then I go into my mail program and open that email and save the invoice on my desktop. then I scan in my documents and save them to my desktop. then I go back into my email program ( not in sage) and make a new email addressed to my customer and attach the copy of the invoice from my desktop along with the scanned in supporting documents all in one email. and then I just say in my email, " please find attached a copy of your invoice and supporting documents for your po #xxx. If you have any issues opening this email please contact our office. Please put in for payment . and then the usual " thank you and my email address! 

Thanks for the response Angela, that's currently what I do to send invoices with backup. The reason I'd like to send it through sage itself is I can create a "pay with credit card" link as some clients prefer to pay with credit card. I just don't know how to create that payment link if I were to send it through outlook itself. 

Send that whole email to yourself with the pay with credit card link in it. then forward that email to your customer along with back up documents attached. ( just remove the FW: from the subject line on the forwarded email so customer thinks it came like that .

Good suggestion for including attachments but messy. It would be so much easier and cleaner for customer if you could attach backup PDF's to sage invoice when it gets created.