I had to go out of the country, so I overpaid my AT&T internet bill so it would cover 2 months and rounded up by ~10 cents to the nearest dollar amount. First

I had to go out of the country, so I overpaid my AT&T internet bill so it would cover 2 months and rounded up by ~10 cents to the nearest dollar amount. First month bill, no problem. Second month bill, no problem. Third month bill should be $amount -credit, nope. They took my credit, listed it as an underpayment and applied a fee. So I go to the store; they can't help with account issues, you have to call. I call, sit through the waiting music, get a rep who get a rep is quite obviously doesn't care. No "Sorry for our obvious billing mistake" or anything. They correct the account and ask if I will pay right now, I decide that I will since I don't trust their system to update in a timely manner. The rep then has the audacity to talk about how AT&T charges a convenience fee to pay via phone but they are going to waive it this time. AT&T fiber and Xfinity cable are the only options in my area.... I still can not understand how they made that error in the first place. It's not like accounting, credits and balances are a new thing. The bill even showed the credit transaction correctly, showing it coming out of the bill balance owed.