roChannel store exists on 3.1, and in-app purchases (one time purchase) can be done on 3.1, as well as outright purchase, and in-place upgrade to a Premium channel (where the pro channel replaces the Lite channel).
However roChannelStore does not return the full set of data that you need to properly implement a monthly or yearly subscription to a content item _within_ the channel, which we call in-channel subscriptions.
In Channel subscriptions have never been tested on 3.1 by our QA team and officially, this capability does not exist on that platform.
That being said, I believe it may be technically possible to use the WebAPI as a work around to get the information needed to properly implement an in channel subscription.
- Joel