Yeah it would be good to have some sort of standard so clients could show payment options from within their interface. I am not very found of NWC into relays because of the not so subtle “commitment” to a specific set of relays. If we think about nostr as a p2p protocol (because permissionless npubs and such), relays should be “mere concierges”, so maybe the client should arrange relay payments in the background for the user instead (you already have a wallet setup for sending zaps after all). As for nostr21, I had some heisenbugs but the payment went through. Turns out it was showing me the wrong relay url in some places of the UI: wss://nostr21.nostr21.com instead of just wss://nostr21.com.