Code

    
    /**
    * when an invoice post is saved update balance and total
    */
    add_action( 'save_post_invoices', function( $post_id, $post, $update ) {

      if ( (defined('DOING_AUTOSAVE') && DOING_AUTOSAVE) || wp_is_post_revision($post_id) || $update ) {
        return;
      }

      $account = get_field('invoice_account', $post_id);

      $users = get_field('account_users', $account->ID);

      if ( $users ) {

        foreach ( $users as $user ) {
          if ( in_array('sendinvoice', $user['account_user_options']) ) {

            $to        = $user['account_user']['user_email'];
            $subject   = 'New Invoice';
            $body      = 'Hi ' . $user['account_user']['user_firstname'] . ', <br><br>A new invoice has been posted to your account.<br><br>Hauss Designs<br>314.427.4955<br><a href="https://haussdesigns.com">haussdesigns.com</a>';
            $headers[] = 'Content-Type: text/html; charset=UTF-8';
            // $headers[] = 'From: RGE Rewards';

            wp_mail( $to, $subject, $body, $headers );
          }
        }

      }