Most of my acquaintances don't keep up with newer technologies, even the young ones. They're still stuck in the .NET and Ruby on Rails days.
To find the data usage, go to My Account > My Services tab > click Xfinity Internet > scoll down (https://customer.comcast.com/MyServices/Internet)
Dispute the Comcast charge, don't pay it, and help her get her service secured.
If this is legitimate it may be because of a virus sending spam from her PC.
How much was her original data cap?
They are not getting paid while they drive around looking for rides.
OP, Don't dilly dally when your zone is called getting on the plane and there should be plenty of room for your stuff. No need for anxiety about it. My experience was a comical rarity.
I've concluded that your carry-on backpack method is the way to go
Do you know what kind of plane you will be on and how full it will be?
Lesson learned - buy tickets directly from the airline.