@numericcitizen No idea if it’s a sustainable model, but I love how Agenda does purchases/subscriptions.
Agenda does offer extra premium features that require an In App Purchase, and that make the app even more powerful. If you decide to purchase the upgrade, you permanently unlock all current premium features across all of your devices. Each of them is described in detail below.
Additionally, any new premium features we add in the 12 months following your purchase are also permanently unlocked.