This morning at Build 2015 when Terry Myerson finally took the stage to cover Windows, one of the first announcements concerned paying for apps. Users are familiar with carrier billing which is convenient for users who don't have credit or debit cards. Now the entire Windows Store will allow users to buy apps via carrier billing.
Carrier billing is great for users who don't have a bank account, but have a cell phone plan. This announcement begins to bring what a unified store would look like. Since apps can be purchased once and installed to all devices, it makes sense that carrier billing would work across devices as well.
No firm details on how carrier billing will happen or what it will look like, but this is just the announcement. Hopefully we'll hear more later today or in the coming weeks.