Override WooCommerce Shipping Cost

functions.php

<?php
add_filter( 'woocommerce_package_rates', 'override_ups_rates' );
function override_ups_rates( $rates ) {
    foreach( $rates as $rate_key => $rate ){
        // Check if the shipping method ID is UPS
        if( ($rate->method_id == 'flexible_shipping_ups') ) {
            // Set cost to zero
            $rates[$rate_key]->cost = 0;
        } 
    }
    return $rates;        
}