I just saw this invoice for the second time, and a different client. It is cleverly designed to look like the official domain renewal notice from your registrar (event though most registrars don't send domain renewal notices). Note the text that says it is for search engine listings (whatever that means, anyway). Notice the "Account number" that is there, even though you have no account with them. If you get these letters, please review them to the appropriate attorney's general - of your state and the state from which they are sent. Since they are engaging in mail fraud, there might be some federal agency that can help.

Note, this one is from a company called DomainUSA.