The Federal Trade Commission is moving forward with an investigation into Apple, according to reporting by the Verge.

The FTC's investigation will focus on the 30 percent fee Apple charges rival subscription services who sign up new users via the company's App Store, sources said.

Streaming services pass those fees on to their users -- meaning Tidal and Spotify's $9.99 introductory tier gets bumped up to $12.99. Meanwhile, Apple Music, once their free trial period is over, will charge users a flat $9.99, since they don't levy the same fees on themselves that they do on competing apps. That's the unfair advantage that caught the FTC's attention.

Streaming services have little recourse when it comes to dealing with the fees. Many users go straight to the App Store for signup, and services aren't allowed to link to their own websites from the App Store to give users a cheaper signup option.

One way to cope is to send out a mass email to all your iOS users -- like Spotify recently did -- and tell them that if they signed up via the App Store, they should unsubscribe, then resubscribe via the streaming service's website.

According to the Verge's sources, the FTC has begun to send subpoenas to music services as it builds its case.