woocommerce custom flat rate charges 28by id 29 function

1function wc_ninja_change_flat_rates_cost( $rates, $package ) {
2
3  // Make sure flat rate is available
4  // '13' is Flat Rate ID (What you Define)
5	if ( isset( $rates['flat_rate:13'] ) ) {
6		// Set the cost to $100
7		$rates['flat_rate:13']->cost = 60;
8		
9	}
10		return $rates;
11}
12
13add_filter( 'woocommerce_package_rates', 'wc_ninja_change_flat_rates_cost', 10, 2 );
14