Usually, when I go to some sort of web-application provider, I must create a login/account with them and register to user their paid services.
But with Solid we expect users to authenticate with thier WebId, meaning users already have an account elsewhere - do we then want them to create yet another account for the specific web-application? Potentially meaning they need to login twice to access the service?
It seems to me that there should be an easy-to-use standard Solid way of linking accounts, such that the web-app service provider can detect a new login from a WebId, inform the user immediately that they are newcomers to the service and would they, please, fill-out the payment details (or select the free plan), fetch as much information as possible from the WebId profile, and then automatically link the WebId account with their own automatically generated account.
Maybe this is all well-known, but perhaps it was worth having this kind of workflow well defined in the Solid “best practice” documentation.