If you do make the changes under the “Online delivery” I have found something else.  If you are creating a new invoice for a customer that already has an account with you and you go to their account DO NOT click on the green button to your right mid-way up “New transaction”, if you do this for some reason it will not have a link for them to pay online in their email.  Go to your left where you see “+ New” and create your new invoice for your customer there. I have found it will send them a link to pay the invoice in their email.   It is a green button call “View invoice” in their email.