How to use non-Laravel package

I use Laravel 5.1 and we use Stripe but now I need to change to checkout.com Checkout.com has one php library: https://github.com/checkout/checkout-php-library

I want to implement into my app. FIrst I run :

composer require checkout/checkout-php-api so I install the library and library is inside vendor/checkout folder

I use OrderController and I create public function checkout:

require_once 'vendor\checkout\checkout-php-library\autoload.php';

use com\checkout;

class OrdersController extends Controller
{


    /**
     * Display a listing of the resource.
     *
     * @return \Illuminate\Http\Response
     */
public function payment() {

  return view('front.checkout');

}

public function checkout(Request $request) {

  $data = $request->all();


$apiClient = new ApiClient('sk_test_aaaaaa-5116-999-9270-999999999');
// create a charge serive
$charge = $apiClient->chargeService();

try {
    /**  @var ResponseModels\Charge  $ChargeRespons **/
    $ChargeResponse = $charge->verifyCharge($data['cko-card-token']);

} catch (com\checkout\helpers\ApiHttpClientCustomException $e) {
    echo 'Caught exception Message: ',  $e->getErrorMessage(), "\n";
    echo 'Caught exception Error Code: ',  $e->getErrorCode(), "\n";
    echo 'Caught exception Event id: ',  $e->getEventId(), "\n";
}



}

Now when I make POST request I get:

FatalErrorException in OrdersController.php line 26: main(): Failed opening required 'vendor\checkout\checkout-php-library\autoload.php' (include_path='.;C:\php\pear')

How to integrate this library into my Laravel project?

update: at frontend I have this code:

<script src="https://cdn.checkout.com/js/frames.js"></script>
  <form id="payment-form" method="POST" action="{{url()}}/checkout">
  {!! csrf_field() !!}

    <div class="frames-container">
      <!-- form will be added here -->
    </div>
    <!-- add submit button -->
    <button id="pay-now-button" type="submit" disabled>Pay now</button>
  </form>

    <script>
    var paymentForm = document.getElementById('payment-form');
    var payNowButton = document.getElementById('pay-now-button');

    Frames.init({
      publicKey: 'pk_test_aaaaaaaaa-000-41d9-9999-999999999',
      containerSelector: '.frames-container',
      customerName: 'John Smith',
      billingDetails: {
        addressLine1: '623 Slade Street',
        addressLine2: 'Apartment 8',
        postcode: '31313',
        email: '[email protected]',
        country: 'US',
        city: 'Hinesville',
        phone: { number: '9125084652' }
      },
      cardValidationChanged: function () {
        // if all fields contain valid information, the Pay now
        // button will be enabled and the form can be submitted
        payNowButton.disabled = !Frames.isCardValid();
      },
      cardSubmitted: function () {
        payNowButton.disabled = true;
        // display loader
      }
    });
    paymentForm.addEventListener('submit', function (event) {
      event.preventDefault();
      Frames.submitCard()
        .then(function (data) {
          Frames.addCardToken(paymentForm, data.cardToken);
          paymentForm.submit();
        })
        .catch(function (err) {
          // catch the error
        });
    });
  </script>