If you want the fantasmic package you have to let them know. It is very difficult to get Fantasmic at Mama Melrose. They probably just booked a regular dinner ressie.
There is actually a separate page on the WDW site for booking the Fantasmic dining package (along with other shows). Unfortunately, the reservation link is not working at this time, and the page comes up telling you to call in.
I would say it is almost a certainty that if you booked online, you booked just regular dining rather than the Fantasmic package.
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