I hope that I do not confuse the sushi charge here but this is what we experienced on the Spirit last week.
We ate in Shogun. We were given the Shogun menu and also the sushi menu. If we ordered off the sushi menu there was a different charge for the sushi that was different than if we ordered off the shogun menu.
I understood that the sushi was all you can eat if it was ordered.
We did not order sushi that evening so I do not know how it would have impacted the cover charge.