Yes, the API keys help you to develop pretty much anything you want using the data in your Podio apps.
You can use the Filter Items API to get data out of Podio and use it with the invoices.
The amount of work to make these external integrations depends on what you want to develop and your space/app data structure.
Also have a look at GlobiFlow to automate things. GlobiFlow is available part of Podio premium.
We are one of the Podio partners and can help you if needed with Podio API development. Feel free to contact us on firstname.lastname@example.org