I took out some loans in April 2010 via the internet with National CS and Fox Enterprises. I have sent the revocation letters several times via email and fax. I can't send them via certified mail, as there's no physical address that I can find. However, I actually received a response from Fox Enterprises asking where I copied and pasted the letter from, AND that they were going to run the transaction anyway.
I haven't actually received a response from National CS, but they keep sending me notices from the same email address.