This came out of the discussion on https://issues.jboss.org/browse/THREESCALE-1378
Invoices with 0 or negative cost remain in "pending" state (assuming that after fixing https://issues.jboss.org/browse/THREESCALE-1378 negative invoices will not be charged), and this means more work for the billing process every time (although such invoices are not something that happens too often).
It needs to be decided, how to handle these invoices. The suggestion is to mark them as "cancelled", or some other state that will prevent selecting them as chargeable
Unfortunately, it's not possible to add the cost to the scope query, as it's a calculated value.