To me that sounds like reinventing the wheel, as it's basically the same process by which online shopping services have integrated for years with payment servic

To me that sounds like reinventing the wheel, as it's basically the same process by which online shopping services have integrated for years with payment service providers (PSPs), which contact customers' banks to do strong customer authentication (SCA), such as 3-D Secure v2, without the need for a blockchain. In fact, I can minimally edit your description to make it an accurate description of checking out using Braintree, for example. «The drop-in UIs is an external wallet that can authorize card transactions securely because it is separate from the merchant's web page itself. When I use my credit card to buy something from a retailer, the PSP will ask my bank to do a SCA to confirm that it is me doing it. That's all that the merchant can do, they can ask the PSP to confirm the transaction is authenticated. Once it's confirmed, a one-time unique token for the transaction is created, and used to execute the transaction. This can work because the browser provides the secure separation between website and wallet. Otherwise, no one would trust an online shop with their credit card details to run transactions.»