I think it’s required. If we’re flipping it to have the nostr zapper broadcast the zap (which we should) you need to cryptographically tie the nostr zapper to the payment proof somehow.

I can see the argument for returning the key explicitly (it’s simple, the initiating app is the payer, kinda), but it feels pretty gross (two copies of a key always bad if you can only have one). Seems cleaner to just pass the message in the original URI?