Why is Xfinity Mobile not removing a deactivated device from my account?

I recently visited an Xfinity retail store to add a single line and device to my account. However, the representative mistakenly added two lines and two devices.

When I brought this to their attention in the store, they assured me the extra line and device would be removed immediately and any associated charges would be fully credited. A few days have passed, but the line and charges are still appearing on my account.

When I called phone support, I was told they couldn't help me resolve this. I do not possess this second device, nor do I have active service for it. Every time I call, they keep giving me the same answer