API Function: Merge Invoices
We want to generate real consolidated invoices.
Once a new invoice is generated, we can catch it with a hook. Then we can check whether a user has other unpaid invoices. If he has, we want to merge the new invoice with a previous invoice. For that to work we need an API function to merge invoices.
Ideally, all admin commands should be available through API. Now there is a feeling that we can do everything through API, but when we start working, we always find some small missing functions here and there.