Welcome!
I'm more of an expert in plkanning trips to WDW, but I imagine they are similar. Why not do the opposite: book the trip as if she wasn't going, then add her.
At WDW, the deal is that you can make changes more than 45days out, no problem (though it get slightly complicated if you need a refund). Less than that and they charges you, but usually they only charge if the cost of your trip goes DOWN.
If you add a third adult toa room, then typically - no matter when you add that third adult, there is some sort of extra adult fee - at mods it used to be $10, at deluxes it was more like $15-$20. It may be slightly more for 2009. But again, even if you have that third person at original booking, you'll get that charge.
Beyond that, if you CANCEL your trip between 45 and I think 7 days - the cancelation fee is $100, a change fee is $50 (but usu. only charged if cost of trip goes down). Last minute changes the CANCEL fee is $200.
TRip insirance costs at least $150 (at least whenI checked) and many have conditions that have to be met to get that refund. Also, I looked recently and WDW has a hurricane cancel policy that if there's going to be a hurricane within a week of your trip, no charge for a cancelation.
All of the above applies to WDW....so ask when you call, but I'm betting it will be abl out the same.
