Based on the facts you provided, it sounds bogus to me too.
If you have their acceptance of your cancellation in writing before any of the new charges incurred, they should not have any standing to charge you anything more (unless you have a contract that states otherwise). I would email them - so you have a record of it - with attachments of prior contacts, stating that you owe nothing and they need to leave you alone. Be polite but firm.
If that fails, dispute the charge with the credit card company.
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Most likely, the answer to your question is within the contract that you signed. Review it to determine when you need to give notice of cancellation, what charges you are liable for, etc. Keep copies of everything!
You need to take a look at the written application/contract you see to what they may or may not be entitled to. But some of these fees and add-ons may be bogus and even violate the Michigan Consumer Protection Act.
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