I had this issue when they changed the “rules” re paying a year or so ago. I paid at my usual time — a few days prior to when the account is closed for the month and new statement run. Theoretically not late except they now require payment much earlier or interest is charged.

I didn’t cotton on until my next statement. A very polite phone call and a long history with them got it all waived.

Good luck with this. Maybe ask for a manager.