Hey there, C3H8.
Allow me to step in for a moment and share some information about integrating third-party apps.
The data that QBO receives is dependent upon what the third-party application sends over to us. In regard to how they import invoices, the process depends per application.
We have a separate forum where you can interact with fellow developers, product experts, and engineers, so I encourage posting this question to our Intuit Developer Community. They have developer tools and resources available to help you find ways on how to do finance charges for past due bills.
This information should you point in the right direction developing a software to sync with QBO.
Stay in touch if you have additional questions about QBO. I'll be around to help you. Wishing you continued success.
