WooCommerce - Filter cash on delivery (COD) by state (or any attribute)

functions.php

<?php
/**
 * Process the checkout.
 * This can be used with any attibute from checkout form.
 * Cash on delivery field. Checking if it is correct and if customer is from Mendoza (state)
 * Note: wc_add_notice is the newest function to send error messages. There are more but they are deprecated.
 */

function k_cod_and_state() {

  // Check if payment method is Cash On Delivery (COD)
	if(isset( $_POST['payment_method']) && $_POST['payment_method'] === 'cod') {
	
	  // If you want to send the product to other address, you can't buy through COD
	  // You have to pay when you receive the products
		if($_POST['billing_state'] === $_POST['shipping_state']) {
		
			// Here we filter if the customer is from the state we have COD available
			// If not, we send an error message
			if($_POST['billing_state'] !== 'Mendoza') {
			  // wc_add_notice is the newest function to send error/success/notice messages
				wc_add_notice(__('Cash on delivery is only available for Mendoza'), 'error');
			}
			
		} else {
			wc_add_notice(__('If you want to pay through cash on delivery, your billing and shipping addresses have to be the same.'), 'error');
		}
		
	}
}

add_action('woocommerce_checkout_process', 'k_cod_and_state');