Can "Buy with ApplePay" sync between a website and an app? At WWDC2016 Apple announced that ApplePay could be used to make purchases in Safari and they've created a JS API to allow this. From what I can see this is only for once-off purchases, it simply authenticates you and charges your card.

Is there any similar tool to access purchases which have already been made via an app which the website owner has created? For example if someone buys my task management app from the AppStore, can they then go to my website and "log in with ApplePay" or "log in with TouchID" and (if Apple confirms they have purchased my app) they can now they can access the web version of the product, customer-only support, etc.
I do not believe it is possible. Apple makes it very clear that, in order to protect your privacy, they cannot see what you buy with ApplePay. So there is no way ApplePay could be used to look up past purchases.