2nd purchase entitles customer to 10% off, how?

After a customer has made a purchase, I want to be able to offer them 10% off the next purchase, how do I automate that?

You can try adding this to the override file for:
/includes/languages/english/checkout_process.php

 // bof: add Discount Coupon to Order
// set the Discount Coupon to be used for coupon_id as ORDER_DISCOUNT_COUPON
define('ORDER_DISCOUNT_COUPON', '1');
define('EMAIL_COUPON_INCENTIVE_HEADER', 'Congratulations! To make your next visit to our online shop a more rewarding experience, listed below are details for a Discount Coupon created just for you!' . "\n\n");
// your Discount Coupon Description will be inserted before this next define
define('EMAIL_COUPON_REDEEM', 'To use the Discount Coupon, enter the ' . TEXT_GV_REDEEM . ' code during checkout:  <strong>%s</strong>' . "\n\n");
define('TEXT_COUPON_HELP_DATE', '<p>The coupon is valid between %s and %s</p>');
// eof: add Discount Coupon to Order

Then add the code in RED to the:
/includes/classes/order.php

    $html_msg['INTRO_URL_VALUE']       = zen_href_link(FILENAME_ACCOUNT_HISTORY_INFO, 'order_id=' . $zf_insert_id, 'SSL', false);

[B]// bof: add Discount Coupon to Order
      global $db;
      $email_order_discount .= "\n\n";
      $coupon_id = ORDER_DISCOUNT_COUPON;
      $coupon = $db->Execute("select * from " . TABLE_COUPONS . " where coupon_id = '" . $coupon_id . "'");
      $coupon_desc = $db->Execute("select coupon_description from " . TABLE_COUPONS_DESCRIPTION . " where coupon_id = '" . $coupon_id . "' and language_id = '" . $_SESSION['languages_id'] . "'");
      $db->Execute("insert into " . TABLE_COUPON_EMAIL_TRACK . " (coupon_id, customer_id_sent, sent_firstname, emailed_to, date_sent) values ('" . $coupon_id ."', '0', 'Admin', '" . $email_address . "', now() )");

      $text_coupon_help = sprintf(TEXT_COUPON_HELP_DATE, zen_date_short($coupon->fields['coupon_start_date']),zen_date_short($coupon->fields['coupon_expire_date']));

      // if on, add in Discount Coupon explanation
      //        $email_text .= EMAIL_COUPON_INCENTIVE_HEADER .
      $email_order_discount .= EMAIL_COUPON_INCENTIVE_HEADER .
      (!empty($coupon_desc->fields['coupon_description']) ? $coupon_desc->fields['coupon_description'] . "\n\n" : '') . $text_coupon_help  . "\n\n" .
      strip_tags(sprintf(EMAIL_COUPON_REDEEM, ' ' . $coupon->fields['coupon_code']));

      $html_msg['COUPON_TEXT_VOUCHER_IS'] = EMAIL_COUPON_INCENTIVE_HEADER ;
      $html_msg['COUPON_DESCRIPTION']     = (!empty($coupon_desc->fields['coupon_description']) ? '<strong>' . $coupon_desc->fields['coupon_description'] . '</strong>' : '');
      $html_msg['COUPON_TEXT_TO_REDEEM']  = str_replace("\n", '', sprintf(EMAIL_COUPON_REDEEM, ''));
      $html_msg['COUPON_CODE']  = $coupon->fields['coupon_code'] . $text_coupon_help;

     //comments area
    if ($this->info['comments']) {
      $email_order .= zen_db_output($this->info['comments']) . "\n\n";
      $html_msg['ORDER_COMMENTS'] = nl2br(zen_db_output($this->info['comments']));
    } else {
      $html_msg['ORDER_COMMENTS'] = '';
    }

// add Discount Coupon to TEXT email
    $email_order .= $email_order_discount;
// add Discount Coupon to HTML email
    $html_msg['ORDER_COMMENTS'] .= "<br /><br />" . $email_order_discount;
// eof: add Discount Coupon to Order
[/B]
    //products area
    $email_order .= EMAIL_TEXT_PRODUCTS . "\n" .

This will give them the coupon code in their order-confirmation email every time they place a new order.
You can adjust this as needed but it gives you an idea of how this could be done ...

Thanks, it will be a while till I try this but looks good.
So every order made at the store will give a code that only that login can use and only once for their next order?