additional charges after downgrading from xFi

I upgraded from 400 Mbps to xFi in October with extra $30. I was told by the chat agent that I would be free to downgrade from xFi to 400 Mbps and removed the extra $30. However, I tried that in December, I was still requested to pay additional ~$30 after I downgraded from xFi to 400 Mbps. My bill included additional equipment rental fees and other fees that did not occur before I upgraded to xFi. It is disappointing to know that once xFi is turned on, its additional fees cannot be turned off even without the service. It is very annoying that the internet billing can only increase for one customer, even if the service does not.