<?php
// Details: https://wordpress.stackexchange.com/questions/321284/override-woocommerce-wc-class-function
// Start copy below, if you want to..

// $total is the cart's grand total, including total shipping cost.
// $cart is the cart object - the same as you'd get with wc()->cart.
function rename_me( $total, $cart ) {
    $cat_ids = array();
    foreach ( $cart->get_cart() as $cart_item ) {
        $cat_ids = array_merge(
            $cat_ids, $cart_item['data']->get_category_ids()
        );
    }

    $cat_id = 39;
    $count = count( $cat_ids );

    // Subtracts the total shipping cost from $total.
    if ( 1 === $count && in_array( $cat_id, $cat_ids ) ) {
        return $total - $cart->get_shipping_total();
    }
    // Returns original $total.
    return $total;
}
add_filter( 'woocommerce_calculated_total', 'rename_me', 10, 2 );