I don’t think a whitelist ability is possible in Nostr btw. Best you could do is encrypt the data and share the decrypt key with a list of users.
Discussion
It's my relay ultimately delivering the data; it should definitely be possible.
Maybe…. But I know that Nostr relays don’t have the same authentication options as, say, Mastodon servers, which are heavily focused on controlling who can access the data.
All it needs to know is who the requester npub is; I'll know if they've paid or not and can return or block the notes.
The reality is, as soon as you sell access to a relay, it's effectively public. So the price should be pretty high 🤷♂️