我在你代码里做了一些小改动。
1.我已经嵌入了所有的购物车计算代码围绕交货范围的基础上，产品库存可用性在一个单独的功能（非挂钩），我可以调用任何地方。

• 我添加并更改了一些钩子：
• 用于显示范围的 checkout 挂接
• 一个checkout钩子，用于添加隐藏的输入字段，值为**$from和$to**，以便在提交订单时发布该数据。
• 将交付范围数据保存在订单元数据中的钩子

所以你的代码应该是：

// The function that calculate the delivery dates range (Not hooked)
function calculate_delivery_range() { 

    $all_items_in_stock = true; // initializing

    // Iterating through cart items (to get the stock info)
    foreach (WC()->cart->get_cart() as $cart_item) {

        # HANDLING SIMPLE AND VARIABLE PRODUCTS

        // Variable products
        $variation_id = $cart_item['variation_id'];
        if( 0 != $variation_id) {
            $variation_obj = new WC_Product_variation($variation_id);
            $stock = $variation_obj->get_stock_quantity();
        } else {
            // Simple products
            $product_id = $cart_item['product_id'];
            $product_obj = new WC_Product($product_id);
            $stock = $product_obj->get_stock_quantity();
        }

        if( $stock <= 0 ){
            // if an item is out of stock
            $all_items_in_stock = false;
            break; // We break the loop
        }
    }

    // Items "in stock" (1 to 4 week days)
    if( $all_items_in_stock ){
        for( $start=0, $count=-1 ; $count < 4; $start++ ){
            $weekdays = date('w', strtotime("+$start days"));
            if( $weekdays > 0 && $weekdays < 6 ){
                $count++;
//            echo date('D j (w)', strtotime("+$start days")).', ';
                if($count == 1){
                    $from = date('D, d.m.', strtotime("+$start days") );
                } elseif($count == 4) {
                    $to = date('D, d.m.', strtotime("+$start days") );
                }
            }
        }
    } else { // One or more Items are Out of stock (14 to 21 week days)
        for( $start=0, $count=-1 ; $count < 21; $start++ ){
            $weekdays = date('w', strtotime("+$start days"));
            if( $weekdays > 0 && $weekdays < 6 ){
                $count++;
                if($count == 14){
                    $from = date('D, d.m.', strtotime("+$start days") );
                } elseif($count == 21) {
                    $to = date('D, d.m.', strtotime("+$start days") );
                }
            }
        }
    }

    ## TRANSLATION ##

    // DAYS IN ENGLISH (Source)
    $days_en = array( 'Mon', 'Tue', 'Wed', 'Thu', 'Fri' );

    // TRANSLATE the DAYS in GERMAN (replacement)
    $days_ge = array( 'Mo', 'Di', 'Mi', 'Do', 'Fr' );

    $from = str_replace($days_en, $days_ge, $from);
    $to   = str_replace($days_en, $days_ge, $to);

    // Return the "from" and "to" values in an array
    return array( 'from' => $from, 'to' => $to );
}

// Displaying the dates delivery range in Cart, Checkout and Order reiceived pages
add_filter ( 'woocommerce_cart_collaterals', 'lieferzeit'); // cart page
add_action ( 'woocommerce_review_order_before_payment', 'lieferzeit'); // checkout page
add_action ( 'woocommerce_thankyou', 'lieferzeit'); // Order recieved
function lieferzeit() {

    // Calling the "delivery date range calculation"
    $days_range = calculate_delivery_range();

    ## DISPLAYING ##
    echo '<i class="shipping_icon fa fa-truck fa-flip-horizontal" aria-hidden="true"></i> <div class="lieferung"> Vorauslichtliche Lieferung </div>  <div class="fromto_date"> ' . $days_range['from'] . ' – ' . $days_range['to'] . ' </div> <div class="tooltip">
  <span class="tooltiptext">Gilt nur bei Lieferungen nach Deutschland.</span></div>' ;

}

add_action ( 'woocommerce_review_order_before_payment', 'include_delivery_range_hidden_checkout_fields');
function include_delivery_range_hidden_checkout_fields() {

    // Calling the "delivery date range calculation"
    $days_range = calculate_delivery_range();

    // Output hidden imput fields with delivery dates range values
    echo '<input type="hidden" name="delivery_range_from" value="'.$days_range['from'].'">
    <input type="hidden" name="delivery_range_to" value="'.$days_range['to'].'">';

}

// Save the "delivery range values" in order meta data
add_action( 'woocommerce_checkout_update_order_meta', 'save_delivery_range_in_order_meta',  100, 1 );
function save_delivery_range_in_order_meta( $order_id ) {

    $delivery_range_from = $_POST['delivery_range_from'];
    if ( ! empty( $delivery_range_from ) )
        add_post_meta( $order_id, '_delivery_range_from', $delivery_range_from );

    $delivery_range_to = $_POST['delivery_range_to'];
    if ( ! empty( $delivery_range_to ) )
        add_post_meta( $order_id, '_delivery_range_to', $delivery_range_to );

}

// Adding Delivery range metabox to Order edit pages
add_action( 'add_meta_boxes', 'add_order_delivery_range_meta_boxe' );
function add_order_delivery_range_meta_boxe(){
    add_meta_box(
        'woocommerce-order-delivery-range-values', __( 'Voraussichtliche Lieferung', 'woocommerce' ),
        'order_delivery_range_values', 'shop_order', 'side', 'default'
    );
}

// Adding content to Delivery range metabox to Order edit pages
function order_delivery_range_values(){
    global $post;

    $from = get_post_meta($post->ID, '_delivery_range_from', true);
    $to = get_post_meta($post->ID, '_delivery_range_to', true);

    echo "<p>$from - $to</p>";
}

• 代码在您的活动子主题（或主题）的function.php文件或任何插件文件中。*

此代码经过测试并正常工作
您将在后端订单编辑页面中获得：

我建议您为函数添加前缀以避免冲突。
例如add_action( 'add_meta_boxes', 'cust_add_meta_boxes' );
如果有一种方法可以存储函数lieferzeit（）的输出，而不会改变输出，那就太好了。
例如，您可以将其存储为 meta数据

$order_id = WC()->order->id;
update_post_meta( $order_id, '_lieferzeit_from', $from );
update_post_meta( $order_id, '_lieferzeit_to', $to );

将函数order_my_custom()修改为：

function order_my_custom()
{
   global $post;  
   $from = get_post_meta($post->ID, '_lieferzeit_from', true);
   $to = get_post_meta($post->ID, '_lieferzeit_to', true);
   echo "Voraussichtliche Lieferung<br> $from - $to";
}