Can't get custom fields of a woocommerce product

i am trying to get the custom fields with this function and multiply by the product price. I am getting the product price but when printing the custom fields, i am only getting zeros.. i dont understand why, i had tryed get_metadata() and get_post_custom() too.. but nothing works

    function filter_woocommerce_cart_product_subtotal( $product_subtotal, $product, $quantity, $instance ) { 
        $fields = get_post_meta(get_the_id(), false);
        $add_on = $product->get_price()*$fields[0] + $product->get_price()*$fields[1] + $product- 
                  >get_price()*0.75*$fields[2];

        return $product_subtotal + $add_on; 
}; 
add_filter( 'woocommerce_cart_product_subtotal', 'filter_woocommerce_cart_product_subtotal', 10, 4 ); 

I have tryed to get single fields too but always getting zeros.

I can get the custom fields in function below and save it in a global variable but when i try to access the global variable in the function above, i get zero again.

function tour_product_add_on_cart_item_data( $cart_item, $product_id, $cart ){
/*
if( isset( $_POST['time_add_on'] ) ) {
    $cart_item['time_add_on'] = sanitize_text_field( $_POST['time_add_on'] );
}
*/
if( isset( $_POST['date_add_on'] ) ) {
    $cart_item['date_add_on'] = sanitize_text_field( $_POST['date_add_on'] );
}
if( isset( $_POST['place_add_on'] ) ) {
    $cart_item['place_add_on'] = sanitize_text_field( $_POST['place_add_on'] );
}
if( isset( $_POST['adult_add_on'] ) ) {
    $cart_item['adult_add_on'] = sanitize_text_field( $_POST['adult_add_on'] );
}
if( isset( $_POST['child_add_on'] ) ) {
    $cart_item['child_add_on'] = sanitize_text_field( $_POST['child_add_on'] );
}
if( isset( $_POST['infant_add_on'] ) ) {
    $cart_item['infant_add_on'] = sanitize_text_field( $_POST['infant_add_on'] );
}
    $product = wc_get_product($product_id);
    $GLOBALS["fee"] = $_POST['adult_add_on']*$product->get_price() + 
 $_POST['child_add_on']*$product->get_price() + $_POST['infant_add_on']*0.75*$product->get_price();
    return $cart_item;

}