Subscribe/unsubscribe user from mailchimp list after purchase
Because MailChimp doesn't want you to just subscribe people without their permission, it makes you put this into a field. 

We have clients, however, who put the subscription opt-in into their Terms of Use agreement. A way around this is to create your own rule:

/**
 * @file
 * Rules integration for the mailchimp lists module.
 */

/**
 * Implements hook_rules_action_info().
 */
function mymodule_rules_action_info() {
  $items = array();
  $items['mailchimp_lists_id_user_subscribe'] = array(
    'label' =&gt; t('Subscribe or unsubscribe entity from a MailChimp list ID'),
    'parameter' =&gt; array(
      'entity' =&gt; array(
        'type' =&gt; '*',
        'label' =&gt; t('Email Address'),
        'description' =&gt; t('The email to subscribe.'),
        'wrapped' =&gt; TRUE,
        'allow null' =&gt; FALSE,
      ),
      'list_id' =&gt; array(
        'type' =&gt; 'text',
        'label' =&gt; t('MailChimp List ID'),
        'description' =&gt; t('The list ID found in the mailchimp dashboard.'),
        'allow null' =&gt; FALSE,
      ),
      'subscribe' =&gt; array(
        'type' =&gt; 'boolean',
        'label' =&gt; t('Subscribe'),
        'description' =&gt; t('True to subscribe, False to unsubscribe'),
      ),
    ),
    'group' =&gt; t('MailChimp'),
    'access callback' =&gt; 'mailchimp_lists_rules_access_callback',
    'base' =&gt; 'mymodule_rules_action_entity_subscribe',
  );
  return $items;
}

/**
 * Action callback: Subscribe an entity to a list.
 */
function mymodule_rules_action_entity_subscribe($entity, $list_id, $subscribe) {
    $email = $entity-&gt;value();

    $choices = reset($field_info['instance']['default_value']);
    $choices['subscribe'] = $subscribe;

    if ($subscribe == 1) {
        mailchimp_subscribe($list_id, $email);
    } else {
        mailchimp_unsubscribe($list_id, $email);    
    }
}


Where mymodule is the name of your custom module. This will provide the rule integration you're actually looking for. 

Be careful, though: This method could get you banned from Mailchimp. It's best to combine it with a double opt-in process to prevent people from reporting subscription abuse.