I just wanted to share my experience buying a ticket for a Versailles Opera performance. The opera site is under Spectacles, and I had to navigate around to find the full calendar of opera performances (including ballet -- performances of any type in the opera house at Versailles). The Spectacles include other performances in and outside the chateau.
I had trouble buying a ticket because the purchase page allowed only delivery in France. I didn't want to take a chance on having it sent to my hotel. I found the email address for the box office (billetterie), and exchanged a few emails with them. First they said to choose no delivery fee on the purchase page, which would get me a PDF ticket via email, but I couldn't find that option.
I ended up calling the box office and was able to buy my ticket with my credit card over the phone. They were very nice.
There is also an Opera Experience membership, which in my case is a good value because I planned two and a half days (not counting morning of departure) to explore Versailles. However, normally you pick up your membership card on the night of the performance. I wanted to visit the chateau, gardens, and outbuildings the day of, and possibly the afternoon before that.
They kindly informed me that I can pick up my membership card at the small store at 3 bis rue de Réservoirs, 11:00 am - 6:00 pm, Monday to Friday. Fortunately my arrival is on a weekday. Thus I’ll be able to use my Opera Experience membership card to visit the chateau, outbuildings and gardens, on all of my days in Versailles.