DVD-by-mail is a business model in which customers rent DVDs and similar discs containing films, television shows, video games and the like, ordering online for delivery to the customer by mail. Generally, all interaction between the renter and the rental company takes place through the company's website, using an e-commerce model. Typically, a customer chooses from a list of titles online and adds titles to a queue. As a customer's requested titles become available, the company sends them out. When the customer is finished with the disc, they mail it back to the company. Background Most companies operate on the following model: The customer joins the rental service, typically through an online e-commerce system and a website, and agrees to abide by a list of conditions, and provides some form of electronic payment (e.g., a credit card number). Once the customer has registered, they create a list of titles they wish to watch, which are ranked by the customer by priority. Titles from the list are mailed to the customer as they become available. The customer watches the films or uses the media and then sends the discs back to the rental company using the mail. Most companies let customers keep the films for as long as they want; customers are, however, limited to a set number of discs out at any one time. Commonly, once a disc is returned, another is sent out. Some companies or plans may have a limit on the total number of movies rented in a month. Memberships are usually billed monthly, and includes postage both ways. Variations exist; for example, some companies also offer video game rentals while others offer music. Redbox allows a user to reserve DVDs or Blu-ray discs online to retrieve and return the DVD at interactive kiosks located in various retail establishments. Netflix began an online streaming program allowing for the online viewing of select movies and TV shows. However, around 4.2 million individuals in the U.S. still rent DVDs via mail from the company. Throttling Given sufficiently fast mail delivery, customers on "unlimited" plans who return their discs quickly enough can receive enough shipments in a month that the company's cost of delivery exceeds the fixed monthly subscription fee, making this type of customer unprofitable. Even below this point, higher volume customers are less profitable than customers who receive fewer discs per month. If these customers become too numerous,