Assuming that you have properly followed all of the Dommes pre-session requirements and that you have reached out to her twice to confirm the session then I would send her a very polite email letting her know that you are cancelling the session primarily to avoid the expense of a flight and hotel expenses (assuming that the flight is refundable or that you can at least get a credit voucher to use toward a future flight.

I think that your email should be respectful and polite and avoid criticism, especially since she may have a good reason for not replying to either of your two previous emails such as illness or a personal issue that is consuming her time and attention. If you are so inclined, you might also note that while you are cancelling this session you would reschedule, and if necessary pay a deposit toward that session as a sign of good faith.

Just my probably flawed opinion but I would not get on an airplane and fly to session without some kind of confirmation and affirmative response from the Domme that the session was a go. Just be respectful and polite in your communication.