Pieter van Oudtshoorn

Good day. We created a web app with several users. We want to allow our users to access the SAGE product they subscribe to. The main objective is to allow the users to quote, convert quote to invoice, invoice and receive payment from within our web app. We do not want to offer a payment or card processing portal, we only want the payee to be able to pay the invoice the same way they would if the invoice was sent directly from SAGE by the user. Where do I start?