Billing stopped working.

Here is an answer from Google Support:

Thank you for contacting Google Play Developer Support and reporting the behavior you're seeing with in-app billing.

We have recently made some changes to our systems, and we are now requiring an app to be published before testing. We are currently recommending to publish your APK to the Alpha channel in order to test licensing, in-app billing, and expansion files. There is no need to create a special testing group in the Alpha channel to test these features, however, the app must be published and not be in draft mode.

We apologize for the inconvenience and are working to update our documentation to reflect these changes.