- cross billing
- Using the signup to one site to trigger automated trials and signups to other sites, usually via pre-checked boxes on the join form.
Cross billing is often used to create a tied sale chain that pays for "free" sites. You join a site for free and provide your credit card, and in the terms there includes notices about "7 day free trial at XYZ site". XYZ site then recurs at $40 to $100 per month unless you cancel inside the 7 day period. As the site owner generally won't draw your attention to the "bonus" site, you are often looking at two or three months of charges before you realize you are getting screwed over.