您的代码中存在一些错误...以下代码将合并购物车项目中的所有追加销售ID（不重复），并将它们显示为唯一的追加销售块：

function show_items_upsells( $limit = '-1', $columns = 4, $orderby = 'rand', $order = 'desc' ) {
    $cart  = WC()->cart;

    if ( WC()->cart->is_empty() ) {
        echo 'nothing';
        return;
    }

    $upsell_ids = $cart_item_ids = array();

    // Loop through cart items
    foreach( $cart->get_cart() as $cart_item ) { 
        // Merge all cart items upsells ids
        $upsell_ids      = array_merge( $upsell_ids, $cart_item['data']->get_upsell_ids() );
        $cart_item_ids[] = $cart_item['product_id'];
    }

    // Remove cart item ids from upsells
    $upsell_ids = array_diff($upsell_ids, $cart_item_ids);
    $upsell_ids = array_unique($upsell_ids); // Remove duplicated Ids

    // Handle the legacy filter which controlled posts per page etc.
    $args = apply_filters( 'woocommerce_upsell_display_args', array(
        'posts_per_page' => $limit,
        'orderby'        => $orderby,
        'order'          => $order,
        'columns'        => $columns,
    ));

    wc_set_loop_prop( 'name', 'up-sells' );
    wc_set_loop_prop( 'columns', apply_filters( 'woocommerce_upsells_columns', isset( $args['columns'] ) ? $args['columns'] : $columns ) );

    $orderby = apply_filters( 'woocommerce_upsells_orderby', isset( $args['orderby'] ) ? $args['orderby'] : $orderby );
    $_order  = apply_filters( 'woocommerce_upsells_order', isset( $args['order'] ) ? $args['order'] : $order );
    $limit   = apply_filters( 'woocommerce_upsells_total', isset( $args['posts_per_page'] ) ? $args['posts_per_page'] : $limit );

    // Get visible upsells then sort them at random, then limit result set.
    $upsells = wc_products_array_orderby( array_filter( array_map( 'wc_get_product', $upsell_ids ), 'wc_products_array_filter_visible' ), $orderby, $_order );
    $upsells = $limit > 0 ? array_slice( $upsells, 0, $limit ) : $upsells;

    wc_get_template( 'single-product/up-sells.php', array(
        'upsells'        => $upsells,

        // Not used now, but used in previous version of up-sells.php.
        'posts_per_page' => $limit,
        'orderby'        => $orderby,
        'columns'        => $columns,
    ) );
}

代码进入你的活动子主题（或活动主题）的function.php文件。测试和工作。

用途：echo show_items_upsells();