Snippet to reduce the shipping Cost for Particular Country and Shipping Method if Shipping class exists in cart. If all the Snippet Classes are available in cart then only snippet will affect the price. (AND Logic)

functions.php

/**
 * Snippet to reduce the shipping Cost for Particular Country and Shipping Method if Shipping class exists in cart.
 * Created at : 15 Jan 2019
 * Updated at : 15 Jan 2019
 * PluginHive Plugins : https://www.pluginhive.com/plugins/
 * Gist Link : https://gist.github.com/varun-pluginhive/0d86c88cbe37b240d313588dbcced366
 */
if( ! function_exists('ph_reduce_shipping_method_cost') ) {
	function ph_reduce_shipping_method_cost( $shipping_costs, $package ){
		$shipping_classes		= array( 'shipping_class_a', 'shipping_class_b', 'shipping_class_c' );	// Shipping Classes that should be in cart to reduce the price
		$price_to_reduce		= 5;																	// Price to reduce
		$destination_country	= "US";																	// Destination Country for which rate need to be reduced

		$canada_post_id = "wf_shipping_canada_post";													// Shipping Method For which rate need to be reduced

		if( $package['destination']['country'] == $destination_country ) {
			// Get Shipping Class from Cart
			$cart_shipping_classes = array();
			foreach( $package['contents'] as $line_item ) {
				$product_shipping_class = $line_item['data']->get_shipping_class();
				if( ! empty($product_shipping_class) )	$cart_shipping_classes[] = $product_shipping_class;
			}

			if( count(array_intersect( $shipping_classes, $cart_shipping_classes)) == count($shipping_classes) ) {
				foreach( $shipping_costs as &$shipping_cost ) {
					if( $shipping_cost->get_method_id() == $canada_post_id ) {
						$shipping_cost->set_cost( (float) $shipping_cost->get_cost() - $price_to_reduce );
					}
				}
			}
		}
		return $shipping_costs;
	}
	add_filter( 'woocommerce_package_rates', 'ph_reduce_shipping_method_cost', 10, 2 );
}