diznefan76, OK, first, it's quite a, you know, small world, as I'm right near your town (in U.Place, but looking at Stadium District for places to live b/c we don't like it here in the relative boonies), and we're going to DL about 2 weeks after you.
And we booked through Costco.
I've saved
this page in my Favorites, and then you click on "existing reservation" down on the right side.
Oh darn, they are down for maintenance tonight, but usually I just put in my reservation number and phone number, and I can see my info, and I know they have a button to make payment. Now, I don't know if you can make smaller, multiple payments, but I know I've seen a payment button.

But it does say to call (800) 225-2024 if you need to do something while they are down.
Last week I decided to downgrade us (thanks to ghastly power bills, over 700 bucks over the last 5 months!!!!) from Paradise Pier and the dining plan (after realizing that it doesn't save any money), to no dining plan and the Carousel, and I called some number I found for Costco Disney travel (I think I called the Costco travel number and chose "disney" from the phone tree), and I was routed directly to Disney. Which was cool. Because they were sweet to me.
Have fun!
So if all else fails, you can just do that