WooCommerce – Add fee based on custom field

I am trying to add a fee to checkout based on whether or not a custom checkbox I’ve added it checked. I am really close to getting this to work, but I’m having issues getting the post values to determine if the fee should be applied or not.

Here is the code I have for my custom field:

add_action( 'woocommerce_after_checkout_billing_form', 'my_custom_fields' );
function my_custom_fields( $checkout ) {


  echo '<div id="message_fields"><h3>' . __('Add a Message') . '</h3>';
    woocommerce_form_field( 'add_gift_message', array(
        'type'          => 'checkbox',
        'class'         => array('gift_message form-row-wide'),

        'label'         => __('5x7 Enclosed Personal Message - $4'),
        'placeholder'   => __(''),
        ), $checkout->get_value( 'add_gift_message' ));

    woocommerce_form_field( 'gift_message', array(
        'type'          => 'textarea',
        'class'         => array('gift_message_text form-row-wide'),

        'label'         => false,
        'placeholder'   => __('Your message'),
        ), $checkout->get_value( 'gift_message' )); 
    echo '</div>';
}

This works, and the fields show up perfectly.

At the bottom of form-checkout.php in my theme, I have added this javascript to update the cart total when the field it checked:

<script type="text/javascript">
jQuery( document ).ready(function( $ ) {

$('#add_gift_message').click(function(){
    if ( $(this).is(':checked') ) { 
        $('#gift_message_field').show();
        var gift_message= true;
    } else { 
        $('#gift_message_field').hide();
        var gift_message = false;
    }
    $.ajax({
        type:       'POST',
        url:        wc_checkout_params.ajax_url,
        data:       $( 'form.checkout' ).serialize(),
        success:    function( response ) {
                if ( response ) {
                        var order_output = $(response);
                        $( '#order_review' ).html( $.trim( order_output ) );
                        $('body').trigger('update_checkout');
                    }
        },
        error: function(code){
        },
        dataType: 'html'
    });
});
});
</script>

This updates the order total and runs the following code (this is where the problem is):

add_action( 'woocommerce_cart_calculate_fees', 'woo_add_cart_fee' );

function woo_add_cart_fee( $data ){
    global $woocommerce;

    if ( isset($_POST['add_gift_message']) )
        $woocommerce->cart->add_fee( 'Personal Gift Message', '4.00', true, 'standard' );

}

I know this part is being called because if I take out if ( isset($_POST[‘add_gift_message’]) ), it adds the fee. What I’m having trouble with is determining if the field has been checked or not – I can’t seem to get the POST values inside of woo_add_cart_fee no matter what I do.

Does anyone know how to get this to work? Thank you!!