Late last year Amazon.com added a new aisle in its store: selling subscriptions to online streaming services like Starz and Showtime as an add-on to a Prime membership. The new offering has proved surprisingly popular, new figures show, demonstrating Amazon’s potential to become a powerhouse in subscription TV.

For instance, Amazon has sold around 500,000 subscriptions to Starz through Prime, roughly four times what Starz has sold directly through outlets like Apple TV, Google’s Chromecast and Roku, according to two media executives with knowledge of the numbers.