Use custom field to calculate shipping fee in Woocommerce - php

Question

I'm building a plugin that allow a user to place an order as a gift for someone else (the beneficiary). The gift is delivered to the address of the beneficiary.
I add new fields after the checkout billing form like this.
add_action('woocommerce_after_checkout_billing_form', 'woocommerce_after_checkout_billing_form');
function woocommerce_after_checkout_billing_form($checkout) {
foreach ($this->get_gift_fields() as $key => $field) {
woocommerce_form_field($key, $field, $checkout->get_value($key));
};
}
function get_gift_fields(){
return array(
'gift_firstname' => array(
'type' => 'text',
'label' => __('Prénom'),
'required' => true,
'class' => array('form-row-first'),
),
'gift_lastname' => array(
'type' => 'text',
'label' => __('Nom'),
'required' => true,
'class' => array('form-row-last'),
),
'gift_country' => array(
'type' => 'country',
'label' => __('Pays'),
'required' => true,
'default' => 'FR',
),
'gift_address_1' => array(
'type' => 'text',
'label' => __('Numéro de voie et nom de la rue'),
'placeholder' => __('Numéro et nom de rue'),
'required' => true,
),
'gift_address_2' => array(
'type' => 'text',
'label' => __('Compléments d\'adresse'),
'placeholder' => __('Appartement, bureau, etc. (optionnel)'),
'label_class' => array('screen-reader-text'),
),
'gift_postcode' => array(
'type' => 'text',
'label' => __('Code postal'),
'required' => true,
),
'gift_city' => array(
'type' => 'text',
'label' => __('Ville'),
'required' => true,
)
);
}
I then have a custom method that calculates shipping using only the country.
I tried the answer from this question, but for exemple the state field appears on the billing form, which make the shipping fees calculation not working.
How can I use my custom fields only, especially the gift_country field, to calculate shipping ?
Thanks!