is this mean to be implemented in nostr clients? or is it just a paywall for websites?
You can also sell digital content with a QR code. Scan it with phone camera to initiate the payment, once confirmed, it will decrypt the content .
The QR code contains encrypted content, price, LN address with cryptographic signature. No database needed.
Paywall Url: https://tinyurl.com/zapit-paywall
Paywall QR code:

Discussion
This one is more like a lightning service, future version will include nostr wallet connect for recurring payments. I might also save purchase content on nostr relay.