I have a room only reservation for the second half of a split stay that I made through CRO and I was able to pay it off in four payments by calling them in (they had to be payments of $35.00 or more). It looks like the website is working now, too, so you might be able to use it if you made the reservation through Disney and not through a
travel agent. I made the first half of our stay through
AAA and have to pay them if I want to make payments, not Disney.