Complaint regarding xfinity customer service rep

I am looking for a corporate email for complaints.  I entered a payment arrangement for my past due balance and requested the payment be made on a specific day.  However the representative entered the payment on their system on a differnt day then what I requested.  I know the calls are recorded and when I called to complain and asked them to listen to the recording they told me there was nothing they could do about the payment it was already submitted.  This is illegal to submit a payment on a day other than what I Gave them permission for.  Their response was I needed to pay the past due anyway so they can’t stop the payment.  Like they are going to force me to make the payment when they require it.  What email or phone no can I use to make a formal complaint regarding this?  Also my bank will charge me fees for returned payment or for me to issue a stop payment who is going to cover that fee?