Being billed at old, higher price after changing plan

Good evening! I changed my Xfinity plan on August 28 (dropped the TV plan and kept internet). I didn't expect for the changes to be immediate, but they were, which was fine. I returned my TV equipment to a local store on 9/1. I had noticed on the app that the amount I was set to be charged was for the old plan. While I was returning the equipment, I asked the young man at the store what the amount was that I'd actually be billed and he gave me an amount that matched my updated plan. Looking tonight, though, the charges that are going through are over $70 higher than what I've been told they'd be. Considering that I didn't even have TV service during most of the last billing period and that we pay a month in advance, I was wondering why my bill hadn't been updated and if I'll receive a credit for next month's bill. Thanks!