I think the ability to mark a ticket as chargeable/billable would be a good addition for break/fix clients.
Perhaps this would lead the way to improving the ticket invoicing workflow. For instance your techs could mark a ticket a billable and close it for it then to show up on a report at the end of the month with all the tickets that need invoicing. Not exactly sure how it'd work, but I think adding the field would start the process?