At the InvoicePlane Wiki you can get help:
InvoicePlane is not a commercial product.
It is developed in our free time, as a hobby. We do your best to fulfill any legal requirements but we cannot make sure that the app is working 100% correct for your particular business.
We neither offer 24/7 support nor any form of SLA or paid help. All support is provided by the community.
InvoicePlane is a free software nonetheless we have several expenses.
Please donate to support us.