Did you know you can actually make payments on your reservation? Can be done by mailing a check or via credit card over the phone. Can pay the whole total or any part of it.
One thing you could do (since you seem to be considering shortening your trip by 1 night) is to figure out what your package costs minus one night lodging and food plan and pay that up anytime you want. Then as it gets closer to the 45 day mark and you want to cancel a day, you can and Disney won't have any of your money.
Or if you prepay it all, when you cancel a night you should instruct the person at CRO you want the remaining balance left on your account (which yes, they can do) as spending money-- otherwise they will seek to refund you via check or credit card (however you paid in the first place).