there is usd based ecash too, see: https://github.com/MakePrisms/boardwalkcash-deprecated

according to the gh link:

> certificate issued to the user, based on the rules agreed with the software vendor.

> certificate is used as a micro-payment for infrastructure resource (it cannot be transferred to any other user without risks of double spend).

> certificate can only be voided by software vendor (or delegate who issued it), so it cannot be used directly as a payment.

they are basically describing ecash, their whole use case is ecash, which is nothing more than issuing a certificate or web token for a payment, and it can be done using monero as payment too

using an "NFT" for a voucher instead just makes it worse, less private, keeps it on the blockchain forever, requires another blockchain, which will definitely not have privacy by default, requires the purchase, usage, download of wallet software, of another token, has extremely worse ux, all that adds a lot more friction, all just to issue an online voucher... which is a solved problem already...