For faster repeat in-app payments (see bug 703073) use PayPal's pre-auth API so that users can make future purchases to a certain app without going through the full PayPal buyflow.
There is a prototype of using preauth at https://github.com/mozilla/apps-payment-server/
However, it's not working right at the time of this writing. It might need to be adjusted for digital goods.
this was fixed as part of bug 739582