Overage Charge

Since I have been charged $10 for 50GB of overage this month for going 7GB over, the remaining 43GB carry over without being charged again. Correct? I have two children who are out of school for the summer and obviously the internet usage has skyrocketed. I have only ever gone over the limit in the summer time, and when school is closed for 2+ weeks at a time. I find it unfair to be charged $10 for “50GB” if I don’t get to HAVE the remaining 43 since it’s already been purchased and added to my account. Especially since I didn’t choose to cancel when my plan increased without notice a few months back. (That in itself was a bit wrong. Notification of some sort should have been at the very least, emailed prior to the extra charge with no warning.)