Billing Time directly to Customer (through activities?)

I am looking for a way to bill time directly to customers. I noticed when creating an Activity for a customer (work item, phone call, etc.), you can check the activity as “Billable”. How do you bill these activities to customers? 

I am guessing the activity checkbox (Billable) is only for reporting on the timecard. 

I am trying to avoid creating a project and invoice for a 15 minute consulting phone call. I’d love to way to track all billable work items for a customer and then bill them at the end of the month / quarter with all these work items. 

Thanks,

Paul