当购物车中没有商品出售时，以下人员将对第二个商品提供10%的折扣：

add_action('woocommerce_cart_calculate_fees' , 'custom_2nd_item_discount', 10, 1);
function custom_2nd_item_discount( $cart ){

    if ( is_admin() && ! defined( 'DOING_AJAX' ) )
        return;

    // Only for 2 items or more
    if ( $cart->get_cart_contents_count() < 2 )
        return;

    // Initialising variable
    $has_on_sale = false;
    $count_items = $discount = 0;
    $percentage  = 10; // 10 %

    // Iterating through each item in cart
    foreach( $cart->get_cart() as $cart_item ){
        $count_items++;
        if( $cart_item['data']->is_on_sale() ){
            $has_on_sale = true;
        }
        if( 2 == $count_items ){
            $discount = wc_get_price_excluding_tax( $cart_item['data'] ) * $percentage / 100;
        }
    }

    // Apply discount to 2nd item for non on sale items in cart
    if( ! $has_on_sale && $discount > 0 )
        $cart->add_fee( sprintf( __("2nd item %s%% Discount"), $percentage), -$discount );
}

代码function.php活动子主题（或活动主题）的文件中。测试和工作。