This is only available at CRT. It should be available at all meals. You may be able to buy them at the podium if you don't plan to eat there.
It is a tall mug...looks like a root beer mug. It is very nice and heavy glass. It is printed with "Cinderella's Royal Table" on the front. When I dined (lunch at CRT 2002) there were two smoothie choices, Raspberry and something that I remember as chocolate??? Sorry if my memory is fuzzy. The drink/mug combo was $8. The drink was $5 and the mug $3. They give you a special reciept when you are done with your meal that you take to the hostess desk as you leave. They then hand you a clean and packaged mug.
I called after returning home and ordered 3 more(at $3 each plus shipping) from the disney merchandise number. I filled them with little trinkets from the parks, and some pixie dust from our trip. I also put some brightly colored streamers inside. They were for my three younger siblings (20, 13, and 10). They were a huge hit...a very nice gift for $3!!! I can't imagine what these would cost in the gift shop??? Oh, we also got the wine glasses (free) as we were there on our honeymoon.