Display total subsidy amount based on WooCommerce cart items shipping class and quantity

With the code below added to my theme's fuctions.php file, I am trying to display a custom total subsidy amount based on WooCommerce cart items shipping class and quantity:

/**
 * Gets the total subsidies amount
 * 
 * @return float
 */
 function get_subsidies_total(){
    $quantity = $cart_item['quantity'];
    $shipping_class_id = $product->get_shipping_class_id(); 
    $shipping_class_term = get_term($shipping_class_id, 'product_shipping_class');
    $subsidio_total = '';

    // WC Cart
    if ( WC()->cart ) {
        // Get cart
        $cart = WC()->cart;

        // Loop through cart items
        foreach ( $cart->get_cart() as $cart_item ){    
            if ($shipping_class_term->slug == 'envio-gratis-o-preferente-125') {
                $subsidio_subtotal = 125;
            } 
            else if ($shipping_class_term->slug == 'envio-gratis-o-preferente-73') {
                $subsidio_subtotal = 73;
            }
            else if ($shipping_class_term->slug == 'envio-preferente-50') { 
                $subsidio_subtotal = 50;
            }
            else if ($shipping_class_term->slug == 'envio-preferente-30') {
                $subsidio_subtotal = 30;
            }
            else if ($shipping_class_term->slug == 'envio-preferente-25') { 
                $subsidio_subtotal = 25;
            }
            else if ($shipping_class_term->slug == 'envio-preferente-12-50') { 
                $subsidio_subtotal = 12.5;
            }
            else if ($shipping_class_term->slug == 'envio-preferente-5') { 
                $subsidio_subtotal = 5;
            }
            $subsidio_row = $quantity * $subsidio_subtotal;
            $subsidio_total += $subsidio_row;
            return $subsidio_total;
        }
    }
}

add_action ('woocommerce_cart_totals_before_shipping', 'mostrar_subcidio', 1);
function mostrar_subcidio(){
    $subsidio= $subsidio->get_subcidio_total();
    echo $subsidio;
}

But it doesn't work.

How can I display the total subsidy amount based on cart items shipping class and quantity in WooCommerce cart and checkout?

There are multiple mistakes and complications in your code, try instead the following compacted and optimized code (commented):

/**
 * Utility function: Get shipping class subsidy amount
 * 
 * @return float
 */
function get_shipping_class_subsidy_amount( $shipping_class ) {
    // return a float number for '12-50' case
    if ( false !== strpos($shipping_class, '12-50') ) {
        return 12.5;
    }
    // Extract the integer from the slug
    else {
        return preg_replace('/\D/', '',  $shipping_class);
    }
}

/**
 * Utility function: Get total subsidies amount
 * 
 * @return float
 */
function get_total_subsidies( $cart_items ){
    $total_subsidy = 0; // Initializing

    // Loop through cart items
    foreach ( $cart_items as $item ){
        $subsidy = (float) get_shipping_class_subsidy_amount( $item['data']->get_shipping_class() );

        if ( $subsidy > 0 ) {
            $total_subsidy += $item['quantity'] * $subsidy;
        }
    }
    return $total_subsidy;
}


/**
 * Display HTML formatted total subsidies amount in classic cart and checkout pages.
 * Hooked function.
 */
function display_total_subsidies() {
    if ( $total_subsidy = get_total_subsidies( WC()->cart->get_cart() ) ) {
        echo '<tr class="subsidies">
            <th>' . esc_html__( 'Subsidies', 'woocommerce' ) . '</th>
            <td data-title="' . esc_html__( 'Subsidies', 'woocommerce' ) . '">' . wc_price($total_subsidy) . '</td>
        </tr>';
    }
}

add_action ( 'woocommerce_cart_totals_before_shipping', 'display_total_subsidies', 10 ); // for Cart page
add_action ( 'woocommerce_review_order_before_shipping', 'display_total_subsidies', 10 ); // for Checkout page

Code goes in functions.php file of your child theme (or in a plugin). It should work with classic cart and checkout pages, but not with newly cart and checkout Blocks (not customizable).