As a matter of fact, I used this scenario to help document how to make an extension. I’ve attached a zip file that contains the files for the module.
Download the zip file and extract it into extensions directory. Next go to the Settings tab and select the Customize menu option. On the Customize screen, select the Extensions option. You should see the cronInvoiceTotal extension. Select the leaf on the left to register the module. Then select the light switch on the left to enable the module. Now if you access the Recurrence screen, the invoice total should be present.
Let me know how this works.