Checkout Coding – Fees Counting Towards Subtotal

We are developing the webpage for our e-commerce business and encounter the following problem. We sell teas and allow customers to choose to have their order in tea bag form for an added fee. Somehow the added fee is separated from the subtotal and thus excluded when applying coupons or calculating free shipping threshold. The code that governs the fee is as below in the functions.php file:

add_action( 'woocommerce_cart_calculate_fees', 'woo_add_cart_fee' );
function woo_add_cart_fee() {

global $woocommerce;

$qtytotal = 0;
$cnt = 0;
$curr_cost = 0;
foreach ($woocommerce->cart->get_cart() as $item) {

$qty = $item['quantity'];
$cost = get_post_meta($item['product_id'], 'customer_wants_tea_bag', false);

if (!empty($cost)) {
$qtytotal = $qty + $qtytotal;
$curr_cost = $cost[0];
}
}
$woocommerce->cart->add_fee( __('Packing', 'woocommerce'), $qtytotal * 6, TRUE);
}

Any idea as to how to change the fee as part of subtotal? Thank you very much in advance.

Kirby