After reading all docs and testing the Subscription API / Selling Plans in our dev store, we have some questions relevant for all of our apps, especially for our Upsell app.
Things we need to know:
1. how to access susbscriptions (so called SellingPlans). We can read them via Liquid and cart.json, but we cannot for example upsell a subscription (cannot get) created by other apps
2. Is there a plan to make subscriptions available in ResourcePicker (we need this to be able to select subscriptions as upsell items )
I wish I had better news for you.
Currently, an app can only access its own selling plans, not those created by other apps. I will make a note that this is a requested feature.
There is also no current plan to make selling plans part of the ResourcePicker. I will also make a note that this was requested.