How can i contact ''Seventeen''?

well "accept any issues already mailed with our compliments" would mean that they are sending them to you for free, so I don't get why you are worried about the bill. They probably just sent one out to you before they had recieved your unsubsciption notice, so they are giving it to you. Its not like they can just go and take the money out of your bank account, so if you do get a bill, they can't make you pay.