Commerce shippping pane that automatically chooses a shipping method without providing a UI for the cutomer

CustomShippingInformation.php

<?php

namespace Drupal\MODULE\Plugin\Commerce\CheckoutPane;

use Drupal\commerce_shipping\Entity\ShipmentInterface;
use Drupal\commerce_shipping\Entity\ShippingMethod;
use Drupal\commerce_shipping\Plugin\Commerce\CheckoutPane\ShippingInformation;
use Drupal\Core\Entity\Entity\EntityFormDisplay;
use Drupal\Core\Form\FormStateInterface;
use Drupal\Core\Plugin\ContainerFactoryPluginInterface;
use Drupal\Core\Render\Element;

/**
 * Provides the shipping information pane.
 *
 * Collects the shipping profile, then the information for each shipment.
 * Assumes that all shipments share the same shipping profile.
 *
 * @CommerceCheckoutPane(
 *   id = "MODULE_shipping_information",
 *   label = @Translation("Shipping information"),
 *   wrapper_element = "fieldset",
 * )
 */
class CustomShippingInformation extends ShippingInformation implements ContainerFactoryPluginInterface {

  protected $eu_countries = [
    "AT",
    "BE",
    "BG",
    "CY",
    "CZ",
    "DE",
    "DK",
    "EE",
    "ES",
    "FI",
    "FR",
    "GB",
    "GR",
    "HR",
    "HU",
    "IE",
    "IT",
    "LT",
    "LU",
    "LV",
    "MT",
    "NL",
    "PL",
    "PT",
    "RO",
    "SE",
    "SI",
    "SK"
  ];

  /**
   * {@inheritdoc}
   */
  public function buildPaneForm(array $pane_form, FormStateInterface $form_state, array &$complete_form) {
    $pane_form = parent::buildPaneForm($pane_form, $form_state, $complete_form);
    $pane_form['recalculate_shipping']['#access'] = FALSE;
    $pane_form['shipments']['#access'] = FALSE;
    return $pane_form;
  }

  /**
   * {@inheritdoc}
   */
  public function validatePaneForm(array &$pane_form, FormStateInterface $form_state, array &$complete_form) {
    $shipment_indexes = Element::children($pane_form['shipments']);

    foreach ($shipment_indexes as $index) {
      $shipment = clone $pane_form['shipments'][$index]['#shipment'];
      $form_display = EntityFormDisplay::collectRenderDisplay($shipment, 'default');
      $form_display->removeComponent('shipping_profile');
      $form_display->removeComponent('title');
      $form_display->extractFormValues($shipment, $pane_form['shipments'][$index], $form_state);
      $form_display->validateFormValues($shipment, $pane_form['shipments'][$index], $form_state);
    }
  }

  /**
   * {@inheritdoc}
   */
  public function submitPaneForm(array &$pane_form, FormStateInterface $form_state, array &$complete_form) {
    // Save the modified shipments.
    $shipments = [];
    foreach (Element::children($pane_form['shipments']) as $index) {
      /** @var \Drupal\commerce_shipping\Entity\ShipmentInterface $shipment */
      $shipment = clone $pane_form['shipments'][$index]['#shipment'];
      $form_display = EntityFormDisplay::collectRenderDisplay($shipment, 'default');
      $form_display->removeComponent('shipping_profile');
      $form_display->removeComponent('title');
      $form_display->extractFormValues($shipment, $pane_form['shipments'][$index], $form_state);
      $shipment->setShippingProfile($pane_form['shipping_profile']['#profile']);
      $this->setShippingMethodByShippingAddressCountry($shipment);
      $shipment->save();
      $shipments[] = $shipment;
    }
    $this->order->shipments = $shipments;

    // Delete shipments that are no longer in use.
    $removed_shipment_ids = $pane_form['removed_shipments']['#value'];
    if (!empty($removed_shipment_ids)) {
      $shipment_storage = $this->entityTypeManager->getStorage('commerce_shipment');
      $removed_shipments = $shipment_storage->loadMultiple($removed_shipment_ids);
      $shipment_storage->delete($removed_shipments);
    }
  }

  public function setShippingMethodByShippingAddressCountry(ShipmentInterface &$shipment) {
    /** @var \Drupal\commerce_shipping\Entity\ShipmentInterface $shipment */
    $shipping_profile = $shipment->getShippingProfile();
    $address = $shipping_profile->get('address');
    $country = $address->country_code;
    $shipping_method_id = 3;

    if ($country == 'CH') {
      $shipping_method_id = 1;
    }

    else {
      if (in_array($country, $this->eu_countries)) {
        $shipping_method_id = 2;
      }
    }

    $shipping_method = ShippingMethod::load($shipping_method_id);
    $shipment->setShippingMethod($shipping_method);
    $shipping_method_plugin = $shipping_method->getPlugin();
    $rates = $shipping_method_plugin->calculateRates($shipment);
    $shipment->setAmount($rates[0]->getAmount());

    return $shipping_method;
  }

}