code: public void onInfiniteGasButtonClicked(View arg0) {
          if (setupDone == null) {
              complain("Billing Setup is not completed yet");
              return;
          }
  
          if (!setupDone) {
              complain("Billing Setup failed");
              return;
          }
  
          if (!mHelper.subscriptionsSupported()) {
              complain("Subscriptions not supported on your device yet. Sorry!");
              return;
          }
  
          /* 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 = "";
  
          setWaitScreen(true);
          Log.d(TAG, "Launching purchase flow for infinite gas subscription.");
          mHelper.launchPurchaseFlow(this,
                  InAppConfig.SKU_INFINITE_GAS, IabHelper.ITEM_TYPE_SUBS,
                  RC_REQUEST, mPurchaseFinishedListener, payload);
      }
docs: flow for subscription.