I assume you can't mention it in the iOS app (it says $12.99 there, no mention of $9.99), but it mentioned in an email I got from YouTube:

http://i.imgur.com/90fJxMS.png

IIRC that's for services and digital products only available inside the app store app, but not for external services and physical goods. This is an external service that happens to also be available via an app.