Actually, you could call them at any time to add the flights: if you called now, you wouldn't know which flight you'd be on at this stage (there's a term for this they explained to me last year, but I can't remember what it was!), but the cost they quote today would be what you pay regardless of whether the actual flights costs more or less when they are released. You could always call and ask for a quote and see what they... and if you're happy, you can book it.
Ooooh I didn't know this, my mum is pranging out about booking our trip without flights so I suspect she'll end up doing this so that we have *a* flight booked on our dates.