Charged with data overage because my son used date while out to sea in the navy

Good morning,
My son is on his first deployment and did know using apps while out to sea would charge a lot in data usage. He is on my plan and we were charged $723.21. When I called the first time all att tied to do was sell me more stuff and told me it wouldn't show up until the next pay period but 1 week later I received a bill saying I had 14 days to pay it. I could not get a hold of my son so I called att back and they put me on extension to pay in 30 days but later that day I got a hold of my son and he paid it even though he can't afford it. Now I still show a payment arrangement on my account for the 723.21 to be paid in 30 days even though it has already been paid and I can't cancel it. My first complaint is that why couldn't att help reduce the amount owed because it was an honest mistake not to mention my son is fighting for this country and barely getting paid. I have been a loyal att customer for over 20 years, always paying on time. Now I want to leave att. my name is Lula I am very unhappy!!!! [EDITED per Community Guidelines]