We were planning on writing a NIP like that for our OrangeCheckr PoC
Testing one-click relay renew popup flow with nostr:npub18kzz4lkdtc5n729kvfunxuz287uvu9f64ywhjz43ra482t2y5sks0mx5sz (NIP coming). If your paid relay subscription has expired, the relay can send this popup to renew your access.

Discussion
We are using a reply with ["PAY", lninvoice, description, link for other options].
The relay can send this at any moment and it will be immediately displayed on the screen.
Nice. Is there any response back beyond the ln invoice being paid? Just like the ["OK, ...] R?response from relays?
Also, do you have a working draft online for this nip?
No response needed. Relay keeps track of the invoice and grants access as soon as it's paid. People can fake payments on Nostr, but they cannot do it on lightning.
Not draft yet. I need to do some final tests with the invoice db nostr:npub18kzz4lkdtc5n729kvfunxuz287uvu9f64ywhjz43ra482t2y5sks0mx5sz has, but it seems to be working.
Awesome! I’ll be home tomorrow evening to add/test anything else you need.