Just an update. Yes, I booked thru a casino offer. It only showed TBA, but I finally found a button that said 'another room'. I still don't know why all the categories were not shown without this step. To be sure, I called to pick a specific cabin. I was told the cabin would be about $330 more than the original TBA. I told him to book it. Then, the Carnival rep told me that my future cruise credit was figured incorrectly when I first booked and I was credited an additional amount equal to about $330. I therefore got my assigned cabin choice for about $10 less than the original TBA.
With Cheers for about $100 more, but probably wouldn't get my money's worth.