Display message if order total is below a certain amount such as to get free shipping after adding products to the cart and when viewing the cart page.

CartEventSubscriber.php

<?php

namespace Drupal\MODULE\EventSubscriber;

use Drupal\commerce_order\Entity\Order;
use Symfony\Component\EventDispatcher\EventSubscriberInterface;
use Drupal\commerce_cart\Event\CartEntityAddEvent;
use Drupal\commerce_cart\Event\CartEvents;
use Drupal\Core\StringTranslation\StringTranslationTrait;
use Symfony\Component\HttpKernel\Event\GetResponseEvent;
use Symfony\Component\HttpKernel\KernelEvents;

class CartEventSubscriber implements EventSubscriberInterface {

  use StringTranslationTrait;

  /**
   * The amount from which orders get free shipping
   *
   * @var int
   */
  private $freeShippingAmount = 250;

  /**
   * Constructs a new CartEventSubscriber object.
   *
   * @param \Drupal\Core\StringTranslation\TranslationInterface $string_translation
   *   The string translation.
   */
  public function __construct(TranslationInterface $string_translation) {
    $this->stringTranslation = $string_translation;
  }

  /**
   * {@inheritdoc}
   */
  public static function getSubscribedEvents() {
    $events = [
      CartEvents::CART_ENTITY_ADD => 'addToCart',
      KernelEvents::REQUEST => 'processPage',
    ];
    return $events;
  }

  /**
   * Displays an add to cart message.
   *
   * @param \Drupal\commerce_cart\Event\CartEntityAddEvent $event
   *   The add to cart event.
   */
  public function addToCart(CartEntityAddEvent $event) {
    $order = $event->getCart();
    $this->displayFreeShippingMessage($order);
  }

  /**
   * Displays an add to cart message.
   *
   * @param \Symfony\Component\HttpKernel\Event\GetResponseEvent $event
   *   The add to cart event.
   */
  public function processPage(GetResponseEvent $event) {
    $method = $event->getRequest()->getMethod();
    $route_name = \Drupal::routeMatch()->getRouteName();
    if($route_name === 'commerce_cart.page' && $method === 'GET') {
      /* @var \Drupal\commerce_cart\CartProvider $cart_provider */
      $cart_provider = \Drupal::service('commerce_cart.cart_provider');
      $carts = $cart_provider->getCarts();
      if(count($carts)) {
        $order = array_values($carts)[0];
        $this->displayFreeShippingMessage($order);
      }
    }
  }

  /**
   * Displays an add to cart message.
   *
   * @param \Drupal\commerce_order\Entity\Order $order
   *   The add to cart event.
   */
  public function displayFreeShippingMessage(Order $order) {
    $total = $order->getTotalPrice();
    $price = new \Drupal\commerce_price\Price($this->freeShippingAmount, 'CHF');

    if($total && $total->lessThan($price)) {
      drupal_set_message($this->t('Orders above @amount get free shipping worldwide!', [
        '@amount' => $price,
      ]));
    }
  }

}

MODULE.services.yml

services:
  MODULE.cart_subscriber:
    class: Drupal\MODULE\EventSubscriber\CartEventSubscriber
    arguments: ['@string_translation']
    tags:
      - { name: event_subscriber }