What does it even mean? You can’t reference an interface. You mean that it’s implemented, surely - and that’s fine. But you’re not sending the message to the right actor instance. This sends this message to this very actor:
An interface call needs an instance as a target, here the instance is Self. This actor sends a message to itself, not to some other (shop) actor.
Note what you did here:
This is fine. You’re sending a message to another actor that you detected. Not self.