Charged a penalty for late return of equipment

I upgraded my modem and installed it myself. The website states I have 30 days to return the old modem. Since I also had some other equipment to return later, I waited until I had all the equipment together to return and took it to my local Xfinity store. The guy there told me I only had 10 days to return equipment and that I was charged a penalty. He said he wouldn't remove it. The store was so crowded I didn't want to waste time arguing with the young man so I accepted his statement that he couldn't remove it, and, that it was charged to "incentivize" prompt return of equipment. I have been an Xfinity customer for years and my bills are auto paid. I did return the equipment within 30 days; is there no way this penalty can be removed?

Thanks for any information.

Carolyn