code: public void onBuyGasButtonClicked(View arg0) {
          Log.d(TAG, "Buy gas button clicked.");
  
          if (mSubscribedToInfiniteGas) {
              complain("No need! You're subscribed to infinite gas. Isn't that awesome?");
              return;
          }
  
          if (mTank >= TANK_MAX) {
              complain("Your tank is full. Drive around a bit!");
              return;
          }
  
          if (setupDone == null) {
              complain("Billing Setup is not completed yet");
              return;
          }
  
          if (!setupDone) {
              complain("Billing Setup failed");
              return;
          }
  
          // launch the gas purchase UI flow.
          // We will be notified of completion via mPurchaseFinishedListener
          setWaitScreen(true);
          Log.d(TAG, "Launching purchase flow for gas.");
  
          /* 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_GAS, RC_REQUEST, mPurchaseFinishedListener, payload);
      }
docs: User clicked the "Buy Gas" button