You can conveniently view the invoice status of your bookings directly from your Bookem Calendar for easy visual reference.
In the image below in Calendar William Tell has been invoiced, but not paid yet, Peggy Carter has been invoiced and has paid and Jack Johnson has not been sent an invoice. To generate an invoice for Jack, simply open the booking and create a new invoice.
You can also see this by going to Calendar and selecting list view at the top right:
How to create invoices and link them to bookings.