The Fantasmic package is not prepaid. It does require a credit card hold and has a cancellation deadline and no-show fee.
There is no "Illuminations dinner show." There is also no restaurant from which Illuminations is visible that will take a credit card hold.
If you're talking about the Candlelight Processional dining package in December (focus of that is the Candlelight Processional show, not Illuminations, although part of the package is a spot where packageholders can watch Illuminations), that is also not prepaid and requires a credit card hold. All Candlelight packages cost 2
DDP TS credits per person, regardless of which restaurant you choose.
In Disney speak, these are not dinner shows, they are dining "packages" including some sort of preferred seating to a show that is otherwise included with park admission. It usually results in earlier entry to the theater to choose seats than the general public is allowed, which can be a good thing with a popular show.
A Disney "dinner show" has both the dinner and the show in one location, and you can't see the show without having the dinner. There are three dinner shows: Hoop de Doo Revue and Mickey's Backyard Barbecue at Fort Wilderness; and the Spirit of Aloha Luau at the Polynesian. All of these would be prepaid at the time of booking.