> If an online retailer already has your payment info and shipping details
This is specifically for the case where the retailer does not want to store these details. Heck they can even get away with not building a user account system at all. Just a store listings page and a few lines of code imported from Stripe.
Totally fair, but this doesn't clarify why a company like Fast needed to exist. It seems the existing payment processors are able to add this feature rather easily, and I don't see how a new player would be able to get over the integration hurdle to supplant PayPay/Stripe/AmazonPay/etc.
This is specifically for the case where the retailer does not want to store these details. Heck they can even get away with not building a user account system at all. Just a store listings page and a few lines of code imported from Stripe.