you can pay cash (or credit card) for it.
No way to use quick service credits at table service restaurant...they don't convert that way, sorry.
The latest Fantasmic Dinner Package reservation we ever got was 5:10 at Mama Melrose for a 9:00 PM Fantasmic show. The earlier the showing, the earlier the latest dinner package. In January, we have a lunch Fantasmic package...the latest dinner they were offering was 3:30, so we just went with a late lunch at 1:30. But then the actual show is at 6:30 so we'll just do dinner later in the evening.