add_filter( 'woocommerce_package_rates', 'add_shipping_option_base_fee', 10, 2 );
function add_shipping_option_base_fee( $rates, $package ) {

    foreach( $rates as $key => $rate ) {
        // reset defaults
        $highest_length = 0;

        // Do not apply base fee if not the right option
        if( $rate->id !== 'betrs_shipping:1-1' ) continue;

        // find highest priority class (BE Table Rate Shipping Only)
        foreach( $package['contents'] as $key => $values ) {
            $item_length = $values[ 'data' ]->get_length();
            if( $item_length > $highest_length ) {
                $highest_length = $item_length;
            }
        }

        // Add $5 surcharge on orders with items larger than 21cm
        if( $highest_length > 21 ) {
            $rates[ $key ]->cost += 5.00;
        }
    }

    return $rates;
}