Unless it's changed, there is no additional charge for the Fantasmic package. You do have to make rezzies at one of three restaurants, either Hollywood & Vine (buffet with a variety of food choices and kid-friendly), Mama Melrose's (pricier & order off the menu), or Brown Derby (priciest but excellent food & order one entree & dessert off the menu). At all three, non-alcoholic drinks are included in the price.
When you call and you get an earlier time than what you want, take it anyhow, and then approx. 30 days from your scheduled arrival, call back and get a later time.
Disney will hold your credit card # until you get there and then charge the card at that time.
If you decide to cancel for any reason, do so before 48 hrs. of that rezzie.
Fantasmic package gets you into the show with no waiting in line. Just about all seats in the reserved section are good except the very end. Try to allow approx. 45 minutes before the show, and although you will be waiting, you will be sitting.
Have a great trip & enjoy the magic!