Product Add-Ons for WooCommerce

We use the Bookings module on our Wellness site for booking holistic treatments. The add-on is great for offering extra treatments, but I would really need it to work on the admin side to be able to take bookings over the phone. Also it adds extra £, but not extra time to the booking. It would be so good if the module could add £5 and 15 minutes to the booking, for example.