I know they would be thrilled if you finished the kit they sent you. The Downy folks are paying for these kits. Buy their products! It's such a nice cause and I bet it serves more and more because folks like you and I are contributing time and talent. Together, we can make a difference.
I think I read the extras would be appreciated, but they're not mandatory. Do what you can. I'm sure anything will be appreciated. They're not going to toss your quilt if you only send one. That would be ridiculous.
If you can't commit to finishing their kit (you'll need backing, batting and basic quilting skills) then you might want to find another way to help out. Or find some friends that have those talents and make one together. Whatever way you slice it, it's a wonderful cause and the Downy folks make it even easier with their generous donations of kits.