I have begun work on an InvoicePlane API. This currently only supports quotes and no authentication…yet.
Stable release will feature the following:
Quotes, Clients, Invoices, Products REST API routes
Sync Customers, Orders, Products and Payments to InvoicePlane
Please feel free to contribute, I have forked the repo here for now:
An OpenCart Extension is also available, but again is still work in progress.
This should be completed soon.
I shall post to this topic once updates have been made.
Any questions, requests etc… please do 
Many thanks,
Adskiremote