The appropriate escalation step here is probably paying a lawyer to send a properly-worded letter on legal letterhead.
As far as dates go, did they use Certified Mail or a similar service? Does the postmark agree with their date or yours? Writing a date on the letter doesn't prove anything about when they sent it.
If it's a scam an not incompetence, a letter from a lawyer will probably shut them up quickly. Litigation usually isn't acquired, you're switching their opponent from a consumer they think they can scam to a professional they know they can't.
As far as dates go, did they use Certified Mail or a similar service? Does the postmark agree with their date or yours? Writing a date on the letter doesn't prove anything about when they sent it.
If it's a scam an not incompetence, a letter from a lawyer will probably shut them up quickly. Litigation usually isn't acquired, you're switching their opponent from a consumer they think they can scam to a professional they know they can't.