At the moment, Billing Reminders are available for new Work Orders created by the Vendor. Soon, Billing Reminders will be available for all Work Orders and you will be able to turn on Billing Reminders for Work Orders. By turning on Billing Reminders, your Vendors get reminders and draft Invoices created for them automatically at the appropriate Invoice frequencies for the appropriate Invoice amounts.
In addition, Billing Reminders will automatically be turned on for Work Orders created in bulk.
Note that 
Invoice Reminders will stop being sent when:
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The Work Order estimated end date has ended
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Or WO has been moved to a completed state.