Killer use case of Cashu. This is how it all started: digital postage stamps.

nostr:note10jxy84xymvp7w20qhqzxwxn6x05ya83jzway86y66nypug990rnsfe46u9  

 what's the difference between this (great) idea and messaging over lightning? 

 Do you mean https://sphinx.chat ? 

Bitcoin lightning network is one financial network, maybe too expensive to be used as normal communication. 

 I tried cipherchat.app but even without it you can send and receive messages with keysend... 

 Can this method also be adopted later to be implemented in the Nostr client for the DM feature, instead of having to install other applications? 

 Yes, that's exactly what Keychat is doing.