I sometimes send bills to companies for money-transfer services. In most cases they don't pay, but when they do, I get free money. My company's website clearly states that it offers services of you transfering money to me by paying my unsolicited bills. Thats the service, if you pay the bill you got the service and thus can't complain. I do offer a 1 month full-refund, just to avoid most lawsuits. I get a lot of repeat orders from large companies with shitty accountants.
Technically not a scam however, cause I've already been challenged in court and I got off with lawyer fees paid.