Part of her catches the way his smile seems to take a pause, but Felicity isn't trained to look for things like that, so she's going off of basic human intuition here. She tries to smile, reassuringly, though she isn't sure what she's reassuring him of exactly.

"A latte," she answers. "Though ... on second thought, maybe I shouldn't have two cups of coffee. Why don't you just get me a peppermint tea instead?"

She doesn't love drinking tea, but it's better than getting too jittery later. Plus, peppermint has a tendency to feel refreshing.