TweetCartel has a lot of different plans, and when you click onto their site, a chat box pops up which connects you to a customer service rep. On the site's "about us" page, they talk about "online and social credibility," and the truth is that a lot of people will just look at your number of followers in determining the weight and influence of your company. It may not seem fair, but so it goes.
This post isn't meant to go into the ethics of buying Twitter followers; it's just to point out the business opportunities in Twitter marketing and the different approaches companies are taking. It takes a long time to build up a large number of followers, and not everyone wants to wait. If you Google this topic, you'll find a lot of interesting reading on the subject.