Originally Posted by Nevarc

It's a direct mail piece. The format is not actually illegal, but it is trickery of the user. Somewhere on the letter it says "This is not a bill".

I wouldn't call it a scam, I'd call it blackhat direct mail. I imagine they made six figures on each mailing since a lot of online companies, my own included get so many invoices per month that a lot of accounts payable departments probably just paid it.