You’ve probably seen online ads with offers to let you try a product – or a service – for a very low cost, or even for free. Sometimes they’re tempting: I mean, who doesn’t want whiter teeth for a dollar plus shipping? Until the great deal turns into a rip-off. That’s what the FTC says happened in a case it announced today. The defendants sold tooth-whitening products under various names, and hired other companies to help them market the products...