That gets a bright smile from Steph, like she's genuinely pleased that Sarah is agreeing to team up with her, which she is, to be fair, but it's also meant to help put Sarah at ease.

"I've been managing, how about you?" Once the flask is full, she tucks it into one of the pouches on her utility belt, since she had the foresight to empty one out for the purpose, "And yeah, I am. Batgirl at your service."

With the flask away, she's free to offer her hand to shake. Even superheroes have manners, sometimes.