The full behaviour may be a bit too much for now, but deciding which to use seems simple: it's how Android Intents already work. Apps register themselves for which kinds of actions they can handle, then the user gets asked which one to use, and can set a default.

> Alexa, we need more bread

< Should I order it through Ocado, Asda, Ebay or Amazon?

> Always use Asda for bread ("unless I tell you otherwise" is implied)