Just discovered IP. Love it. You’ve done a fantastic job.
Now, I’d like to integrate it further with my existing systems - specifically I run an UberCart (Drupal) shop, and I would love to be able to connect that to IP so when a sale is made it generates an invoice, customer, guest account, etc and ties them all together.
I am sure I could just directly manipulate the database backend, but that’s nasty IMHO. So I’d like to investigate expanding (through a module?) IP to connect to the UC database or expose an interface that UC could communicate with.
Is there any development documentation for writing modules? Is there any form of existing API for other systems to communicate with IP?
glad you like it. For InvoicePlane v1 there’s little to no API. For module check out one of the open PR’s where someone made a garage management tool (I think), it’s a module.
For now I think you need lots of manipulation in Codeigniter & ubercart to make it work
I am not familiar with Codeigniter, so I guess I have a bit of reading to do. Something to keep me occupied during lockdown I guess 
Maybe the more useful module to write would be a REST API module or something like that which could allow integration with a wider range of third party shopping carts, etc.
It would be, but we’re creating InvoicePlane V2 reallly soon, so an API for V1 would be a waste of time
And is the intention to have an API in there? Or an easier extensibility system? If so I’ll hold off on any development until that’s ready so I can play with that instead.
We will all thinking about this together, with the entire community, so you can pitch in as well.
The idea that I will be pitching is API-driven, yes, so you can easily extend it.
That way it will also be easier to receive an order from ubercart and convert it to an invoice
That will be perfect. I’ll back your pitch certainly 
API FTW.