Should I have to pay her?

So I'm babysitting right now. The kids' aunt called them and asked what they wanted for lunch because she's going to bring them fast food.

She did not ask me if I wanted anything, which is fine, I don't expect her to, but she brought me a coffee. Of course she did not have to do that and I appreciated it, but then she tells me I owe her like $3.50 for it.

I told her that although I appreciate the gesture, I'm not going to pay for something I did not ask for and I don't have money with me to pay her anyway (even though I do). She started getting mad saying that she did not have to bring me a coffee, but she did, so I need to pay her back for it. She said since I did not pay her for it, she's going to talk to the kids' mom and have her take it out of my pay.

I have been watching these kids for 5 years and have never had anything like this happen before. I'm certain their mom will side with me, but I want to hear your opinions.