Flex box already returned

Flex box was returned months ago when we upgraded our modem because we never once used it.. We recently fully switched to a different carrier and returned the upgraded modem. I'm now getting messages from Xfinity saying I need to return the flex box but this was already returned.  I spoke to a chat agent about a week ago and explained the situation to him. He looked into it and said he had put a note on my account to ensure I would not get charged for this and not to worry. I'm still receiving these messages though and worried I'm going to get charged. I'm not sure what I need to do- Thank you.