Mage2\Checkout\Controllers\CheckoutController::postShippingOption PHP Method

    public function postShippingOption(ShippingOptionRequest $request)
    {
        $orderData = Session::get('order_data');
        $orderData['shipping_method'] = $request->get('shipping_option');
        Session::put('order_data', $orderData);
        return redirect()->route('checkout.step.payment-option');
    }