Unreturned equipment fee but I returned my equipment!

I canceled my service about two months ago and ever since then I’ve kept getting notifications telling me to return my equipment, even though I returned it the day after I canceled my account. I’ve talked to two agents about this, once on the chat and once on the phone, and both of them told me it was fixed. But I just got a bill for a little over eight dollars. The first agent told me they would refund my account $46 or so, which they did, but the fee for the “unreturned equipment” is $55, so now I have a bill for a little over eight dollars. Can you please fix this for good? I’m getting tired of it. Thank you.