The Quickbooks Online Payments were actually very easy to set up. A quick questionaire where you fill in your bank info, a day or so later, an email letting you know you're good to go. The Quickbooks actually assigns the LINK to each individual invoice. If a customer receives online billing, they just click the link. If they receive a printed invoice - their own link is there, just type it into the search bar online, and the rest is up to them.
My customers that are using it are "older" people, so it must be easy enough!