We are trying to understand the relation between the BILLABLE checkbox and the FREE checkbox in the field service module. There doesn't seem to be much content on this through help.  

We have noticed a few issues and not sure if this is a bug or our limited understanding of the workflow. 

Example, we need to return to a customer site to perform a warranty service call:

On Appointment or Service order if the BILLABLE box on any line is UNCHECKED (meaning we don’t want to invoice the customer for the line) the billing will not go through and will hang up in the RUN SERVICE ORDER/APPOINTMENT BILLING process screen indefinitely. (This might be a bug). 

However, if you go back and CHECK the BILLABLE item and ALSO check FREE, the system will generate a $0 SO invoice and will disappear from the process screen.  This creates a second problem; THEN, because there is no way to mark a SO invoice as DO NOT EMAIL (like you can an AR invoice), the $0 SO invoice will sit in a PENDING EMAIL or PENDING PRINT status indefinitely and force you to email a $0 invoice to a customer before it can be released and THEN it will sit in an OPEN status on the account until manually closed.