You can edit the text on a Service/Activity Code simply by going to File Maintenance > Tools > Manage Codes, choosing Service/Activity Codes, and editing the description of the Code you want.  You could edit your Flat Fee codes to have the Words “Flat Fee” at the end of the flat fee text if needed.  This would show on the Invoices and on the Statement of Account report.

Note it is not recommended that you edit Invoice Styles provided by Orion. If you want to edit an Invoice Style, it is recommended you make a copy first by calling it up and doing saving it first under another name.

You could also, if needed, edit your Invoice Style not to print zero hours digit on the detail line item. You may click on the variable for the detail line for the fee hours, right click and choose Blank when Zero. This would suppress the zero from showing. You would need to confirm that you did not have regular fee line items that show as zero hours on the same Invoice, or this method would not be suitable for what you want to do.

Note that editing Invoice Styles is considered a billable service by Orion.