Add mailchimp subscribe to woocommerce checkout.

woo-mailchimp_subscribe.php

<?php
/**
 *  Subscribe User to MailChimp
 */
function woo_mailchimp_subscribe_user($order_id,$posted){
    if(!empty($_POST['wooms_susbscribe'])  && $_POST['wooms_susbscribe'] == '1'){
        try{
            $email = $posted['billing_email'];
            $merge_vars = array('FNAME' => $posted['billing_first_name'],'LNAME' => $posted['billing_last_name']);

            $api = new MCAPI(mailchimp_api_key);
            add_post_meta( $order_id , '_customer_opted_in', 'yes');

            $retval = $api->listSubscribe( mailchimp_listid, $email, $merge_vars, $email_type='html', mailchimp_double_optin);

            add_post_meta($order_id, '_mailchimp_subscribe_result', $retval ? "true" : "false");

        } catch (Exception $e) { add_post_meta($order_id, '_mailchimp_error', json_encode($e)); }

    } else if( !empty($_POST['wooms_susbscribe'])  && $_POST['wooms_susbscribe'] == '0' ){
        add_post_meta( $order_id , '_customer_opted_in', 'no');
    }
}
add_action('woocommerce_checkout_order_processed','woo_mailchimp_subscribe_user',10, 2);

/**
 *  Display Newsletter Checkbox on Checkout
 */
function woo_add_newsletter_checkbox_to_checkout(){
    ?>
<p class="form-row">
	<label class="woocommerce-form__label woocommerce-form__label-for-checkbox checkbox">
		<input type="checkbox" class="woocommerce-form__input woocommerce-form__input-checkbox input-checkbox" name="romani_susbscribe" id="romani_susbscribe" value="1" checked="checked">
		<span class="romani_label"><?php _e("Yes, I'd like to receive email updates and special offers!","romani");?></span>
	</label>
</p>
<?php
}
add_action('woocommerce_checkout_before_terms_and_conditions','woo_add_newsletter_checkbox_to_checkout',99);