code: public void onUpgradeAppButtonClicked(View arg0) {
          Log.d(TAG, "Upgrade button clicked; launching purchase flow for upgrade.");
  
          if (setupDone == null) {
              complain("Billing Setup is not completed yet");
              return;
          }
  
          if (!setupDone) {
              complain("Billing Setup failed");
              return;
          }
  
          setWaitScreen(true);
  
          /* TODO: for security, generate your payload here for verification. See the comments on
           *        verifyDeveloperPayload() for more info. Since this is a SAMPLE, we just use
           *        an empty string, but on a production app you should carefully generate this. */
          String payload = "";
  
          mHelper.launchPurchaseFlow(this, InAppConfig.SKU_PREMIUM, RC_REQUEST,
                  mPurchaseFinishedListener, payload);
      }
docs: User clicked the "Upgrade to Premium" button.