with free accessible rss but private (like public quotes and invoice) we can integrate some invoiceplane feature with service like zapier or ifttt.
example:
rss for accepted quotes => ifttt can pull information from that and send it in a excel or to pushbullet
rss for rejected quotes => ifttt can pull information from that and make blink your blink1
rss for payed quotes => ifttt can pull information from that and send a message to slack
and those are some examples… ifttt and zapier are very powerfull!