The email markup that you're using is the correct way to send the confirmation email for a flight reservation. However, once the passenger has checked in and a boarding pass has been issued, another email needs to be sent with the boarding pass information in order to trigger a card in Google Now.
As long as the confirmation numbers and passenger details match, the two emails will be merged into one flight reservation with a boarding pass. In order for that boarding pass to show up in Google Now you will need to make sure all the required properties are filled in and provide a URL to a barcode/qrcode image using the ticketToken property. Also make sure that the departureTime is sometime within the next 6 hours or change the time on your device to within 6 hours of the specified departureTime.