As I look back over this thread, it seems that the question about mimosas isn’t the real question. The OP wants to know if he can order expensive champagne and the answer is, you can order champagne by the glass and you may have it included in your package, or it may incur an up charge. It might not be available at every bar.
having ordered it, you can also order whatever, mixin you choose, from orange juice to cranberry juice to kir.