Itemized Receipt

I had made payment online and they sent me the usual receipt of payment via email. However, the payment I made 173.13 cannot be found on any of my billings. I asked for an itemized receipt showing exactly what I paid and the only answer I can get is "past due billings". Past due billings show a monthly bill of $520.00. I spent an hour on the phone today with customer service and they cannot tell me how they got $173.13 or what those charges even include. How can you tell someone they owe you money and not even be able to tell you what for????