How to contact comcast over email for a refund

I am just coming out of literally a 4 hour chat session with like 15 agents who kept disconnecting, about a billing refund. Where do I send a refund request over email, so I can attach the chat history?
I have been overpaying for months and months for a modem that I had already sent back. It also needs to be taken off my current account which also seemed to be something the agents couldn't do. 
Thanks!