How to add fees based on product categories and quantity in WooCommerce

I have some product in WooCommerce and two categories are:

Name: ORG Test
Slug: org-test

Name: ORG Prod
Slug: org-prod

I want to calculate shipping fee per quantity ($15 per quantity) if the product matches org-prod category:

My code attempt:

add_action('woocommerce_cart_calculate_fees', 'add_fees_on_ids');
function add_fees_on_ids() {
    $total_act_fee = 0;
    $business_plan_exist = false;
    if (is_admin() && !defined('DOING_AJAX')) {return;}
    foreach( WC()->cart->get_cart() as $cart_item ) {
        $product = $cart_item['data'];
        $quantity = $cart_item['quantity'];
        $categories = wc_get_product_category_list(  $product->get_id() );
        if (strstr($categories, 'org-prod')) {
            $business_plan_exist = true;
            $total_act_fee = $total_act_fee + 15;
        }
        
        if ($business_plan_exist) {
            WC()->cart->add_fee(__('Shipping Fees '), $total_act_fee);
        }
    }
}

But this does not give the desired result. The fee is applied but the total is wrong? Can you assist in figuring out why not?

Solution

Your code contains some mistakes and/or could be optimized:

When calculating the totals, you do not take into account the quantity
Your if condition for adding the fee, is in your foreach loop, so it is overwritten every loop
The use of WC()->cart is not necessary, as $cart is already passed to the callback function
Use has_term() versus wc_get_product_category_list() and strstr()

So you get:

function action_woocommerce_cart_calculate_fees( $cart ) {
    if ( is_admin() && ! defined( 'DOING_AJAX' ) )
        return;
    
    // Specific categories: the term name/term_id/slug. Several could be added, separated by a comma
    $categories = array( 'org-prod', 'categorie-1', 83 );

    // Initialize
    $total_act_fee = 0;
    $amount = 15;
     
    // Gets cart contents
    foreach ( $cart->get_cart_contents() as $cart_item ) {
        // Has certain category     
        if ( has_term( $categories, 'product_cat', $cart_item['product_id'] ) ) {
            // Get quantity
            $quantity = $cart_item['quantity'];
        
            // Addition to the total
            $total_act_fee += $amount * $quantity;
        }
    }

    // Greater than
    if ( $total_act_fee > 0 ) {
        // Add fee
        $cart->add_fee( __( 'Shipping Fees', 'woocommerce' ), $total_act_fee, true );
    }
}
add_action( 'woocommerce_cart_calculate_fees', 'action_woocommerce_cart_calculate_fees', 10, 1 );