Add a checkbox to WooCommerce order form to process MailChimp API 3.0 list member subscriptions (placed after T&C checkbox if enabled on order form)

functions.php

<?php
//add action to process mailchimp signup if checked
function mc_checkout_field_process() {
		//only run the following if the option to subscribe has been selected
		if ( $_POST['mc-subscribe'] == 'on' ) {
				//retrieve email, first, and last name from form
				$email = $_POST['billing_email'];
				$first_name = $_POST['billing_first_name'];
				$last_name = $_POST['billing_last_name'];

				// create a new cURL resource
				$ch = curl_init();

				//unique list and API key values
				$listid = '00-your-list-id00';
				$apiKey = '99-your-api-key99';

				//create our array of values for the subscription
				$data_json = json_encode([
				        'email_address' => $email,
				        'status'        => 'pending', // "subscribed","unsubscribed","cleaned","pending" - pending automatically triggers double opt-in
				        'merge_fields'  => [
				            'FNAME'     => $first_name,
				            'LNAME'     => $last_name
				        ]
				    ]);

				// set URL and other appropriate options
				curl_setopt($ch, CURLOPT_URL, 'https://usXX.api.mailchimp.com/3.0/lists/'.$listid.'/members/');
				curl_setopt($ch, CURLOPT_USERPWD, 'user:' . $apiKey);
				curl_setopt($ch, CURLOPT_HTTPHEADER, ['Content-Type: application/json']);
				curl_setopt($ch, CURLOPT_RETURNTRANSFER, true);
				curl_setopt($ch, CURLOPT_TIMEOUT, 10);
				curl_setopt($ch, CURLOPT_POST, true);
				curl_setopt($ch, CURLOPT_SSL_VERIFYPEER, false);
				curl_setopt($ch, CURLOPT_POSTFIELDS, $data_json);

				// grab URL and pass it to the browser
				curl_exec($ch);
				// close cURL resource, and free up system resources
				curl_close($ch);
		}
}
add_action('woocommerce_checkout_process', 'mc_checkout_field_process');

//save value to order record if checked
function mc_checkout_field_update_order_meta( $order_id ) {
    if ( ! empty( $_POST['mc-subscribe'] ) ) {
      update_post_meta( $order_id, 'mail-chimp-opt-in', 'Yes');
    } else {
	update_post_meta( $order_id, 'mail-chimp-opt-in', 'No');
    }
}
add_action( 'woocommerce_checkout_update_order_meta', 'mc_checkout_field_update_order_meta' );

style.css

/*align mailchimp opt-in on checkout form*/
div.form-row div.mc-subscribe {
  clear: left;
}
div.form-row div.mc-subscribe input.mc-input-checkbox {
  float: left!important;
  width: auto!important;
  margin: 5px!important;
}
div.form-row div.mc-subscribe label.mc-checkbox {
  display: inline!important;
}

terms.php

<!--add this <div> to the existing <p class="form-row terms wc-terms-and-conditions"> element in the terms.php WC template-->
<div class="mc-subscribe">
	<input type="checkbox" class="mc-input-checkbox" name="mc-subscribe" id="mc-subscribe" />
		<label for="mc-subscribe" class="mc-checkbox">Sign Up for Updates and Specials</label>
</div>

Thank you for that! Works perfect!

Great - I'm glad you could use it!