MS Payments might fit in nicely with Windows (and Office) as a service your rent annually rather than buy.
Stranger things have happened.
After all, MS will already have your Microsoft User and Microsoft Account details. Would make perfect sense to associate a Credit/Debit Card number with it. Or a bank a/c.
My money's on MS moving to a monthly/yearly licence platform for Windows, a la Adobe, Office 365 et al, once Windows X has a sizeable user base.
After all. You wouldn't want to be running an insecure, unpatched, non-updated Windows operating system would you? Didn't what happened to those poor saps who stayed wih XP warn you? (oh!! - that hasn't happened yet, has it. But I bet it does. Just at the time Windows X goes subscription based ;-)