Snippet to adjust WooCommerce Shipping Rates to shipping method by specified amount of percent. PluginHive Plugins : https://www.pluginhive.com/plugins/

functions.php

/**
 * Snippet to adjust WooCommerce Shipping Rates to shipping method by specified amount of percent.
 * Created at : 09 Nov 2018
 * Updated at : 09 Nov 2018
 * PluginHive Plugins : https://www.pluginhive.com/plugins/
 * Gist Link : https://gist.github.com/varun-pluginhive/ddba44d01e406891540263eb9264dd0d
 */
add_filter( 'woocommerce_package_rates', function( $package_rates ){

	$cost_adjustment = 15;												// Cost adjustment in percent
	$shipping_method = array( 'wf_multi_carrier_shipping' );			// Shipping method on which adjustment need to be done

	foreach( $package_rates as $package_rate ) {
		if( in_array( $package_rate->get_method_id(), $shipping_method ) ) {
			$cost = (double) $package_rate->get_cost();
			$cost = $cost + ( $cost * $cost_adjustment / 100 );
			$package_rate->set_cost($cost);
		}
	}
	return $package_rates;
});