Although the question seemed innocent enough, the answer was a little trickier than most might think at first glance. Charlie, by very definition, wasn't much of a drinker. She visited a great many bars - and on special occassions (such as New Year's Eve) she'd tie a few on. But for the most part, she didn't allow herself to drink very much. Drinking lowered inhibitions, and Charlie without her inhibitions was often disastrous.
However, if she asked for something else instead, she ran a significant risk of insulting him. Considering she would rather not do that - and considering asking Lucifer for a glass of water seemed almost like it'd be in poor taste anyway - she ultimately decided his selection would suffice.
"Champagne is fine," she replied, "thank you." She didn't have to drink a lot, after all, and she was supposed to be enjoying herself at least somewhat. That's what the instructions had said.
"This is quite a nice place," the blonde added after a second, her own manners dictating she at least make some sort of small talk. It was usually considered rude to just jump right into business and, even if it weren't, Charlie wasn't much of a negotiator anyway. If the Senior Partners wanted someone professional, she wasn't the one they would be using.