Let your users browse and buy add-ons themselves

The account has a catalogue with search, categories and a detail page per add-on, listing what it already has first, with filters for active, acquired and to buy. Free ones switch on with a click; paid ones join the subscription and are invoiced right away. Cancelling one, switching it off or moving to a smaller tier is refused while the account would be left using more than it allows, so nobody loses room for what they already have.