Similar booking flights as part of a Disney package. I assume the rules are different for packages (because ATOL?).
But ... You may find you don't get access to be able to select seats on the plane until you have paid for the flight.
(I had this with a Disney package in 2018 and paid for the flight part earlier to get that access.)
Or (usually/often) if you have an upgraded seat like Premium Economy.but you'll have to pay extra unless you have frequent flier status or a baby on the booking.
Or (usually/often) if you have an upgraded seat like Premium Economy.