This is a Q4 goal. More bugs, specs, and prototypes to follow shortly.
Spec and prototype https://github.com/mozilla/apps-payment-server/blob/master/DESIGN.md
ptheriault, cc'ing you in case you want to get familiar with the in-app payments threat model. See spec / outline above.
This was done in bug 834410
Whoops, scratch that last comment. This is an old AMO tracker. We are tracking payments in bug 775802