Missing Payment on my Account

I have contacted Comcast directly with this issue & even faxed required documentation to the 866-617-4186 number they gave me and have yet to hear a response. Here is what happened (and its a very simple issue that doesn't seem to be getting fixed:

I have multiple accounts with Comcast since there are many different locations within the Park District I work at. I sent payment for 3 accounts on one check since they were all due at the same time. I forgot to send the payment stub on one of the accounts so that payment was not applied to the account, or any of my accounts for that matter.-it is just lost somewhere! I called comcast to inquire about the $10 late fee on that account and they said its on there because payment was never received, eventhough I have proof that I sent it!

My issue doesn't seem to be getting resolved so I'm turning to the forums for help.

Thanks!

-Emily