Returned Hub but still charged – need advice

Hi everyone, I stopped using Virgin Media broadband few months ago. They asked me to return the WiFi Hub, so I ordered the return kit and sent it back through Yodel. However, I’ve still received a bill for not returning the equipment, i paid for that. The problem is that the serial number sticker on my Hub had come off, so I’m not sure if Virgin Media will recognise it as my return. Has anyone had a similar experience? Will Virgin refund me once they confirm the Hub is returned, even without the serial sticker? Thanks in advance for any advice!