limelites:
Thanks for getting back to me. My present checkout success page looks like this:

Just a normal page, nothing fancy, but worded as though someone's just purchased a product, not as though they'd paid an instalment.
I basically want to use different text, nothing fancy, just text that thanks them for their instalment and informs them that their balance will be updated shortly.
I don't know how you'd update the balance automatically, that's way outside the scope of my skills but if you could come up with a solution to that then it'd definitely be well worth adding to the contributions area.
This is all pretty simple to change. I believe that some of it is in a define page section and the rest in the language files. Basically, look at the tpl file and figure out what parts you do not want in the customized page, it's all pretty straightforward.
If it helps, have the checkout success page open in a browser so you can see what the different section of codes look like on the rendered page. It looks like you have product notifications off, do you want the gift certificate section displayed?, etc.
If everything else is turned off then it's really easy to hardcode some text into the template or if you want to get more proper you can add some new defines to the language file. Just do something like this(this is taken from the default tpl_checkout_success_default.php in 1.3.9h):
<?php
/**
* Page Template
*
* Loaded automatically by index.php?main_page=checkout_success.<br />
* Displays confirmation details after order has been successfully processed.
*
* @package templateSystem
* @copyright Copyright 2003-2010 Zen Cart Development Team
* @copyright Portions Copyright 2003 osCommerce
* @license http://www.zen-cart.com/license/2_0.txt GNU Public License V2.0
* @version $Id: tpl_checkout_success_default.php 16313 2010-05-22 08:15:39Z wilt $
*/
?>
<div class="centerColumn" id="checkoutSuccess">
<?php if ($products->fields['products_id'] == '1652') { ?>
<h1 id="checkoutSuccessHeading"><?php echo "Your new heading title"; ?></h1>
<!--(if you want the order number displayed keep the next <div> section, if not delete it) -->
<div id="checkoutSuccessOrderNumber"><?php echo TEXT_YOUR_ORDER_NUMBER . $zv_orders_id; ?></div>
<div id="checkoutSuccessMainContent" class="content">
<!--Your formatted html message goes here. -->
</div>
<!--bof logoff-->
<div id="checkoutSuccessLogoff">
<?php
if (isset($_SESSION['customer_guest_id'])) {
echo TEXT_CHECKOUT_LOGOFF_GUEST;
} elseif (isset($_SESSION['customer_id'])) {
echo TEXT_CHECKOUT_LOGOFF_CUSTOMER;
}
?>
<div class="buttonRow forward"><a href="<?php echo zen_href_link(FILENAME_LOGOFF, '', 'SSL'); ?>"><?php echo zen_image_button(BUTTON_IMAGE_LOG_OFF , BUTTON_LOG_OFF_ALT); ?></a></div>
</div>
<!--eof logoff-->
<br class="clearBoth" />
</div>
<?php } else { ?>
<!--bof -gift certificate- send or spend box-->
<?php
// only show when there is a GV balance
if ($customer_has_gv_balance ) {
?>
<div id="sendSpendWrapper">
<?php require($template->get_template_dir('tpl_modules_send_or_spend.php',DIR_WS_TEMPLATE, $current_page_base,'templates'). '/tpl_modules_send_or_spend.php'); ?>
</div>
<?php
}
?>
<!--eof -gift certificate- send or spend box-->
<h1 id="checkoutSuccessHeading"><?php echo HEADING_TITLE; ?></h1>
<div id="checkoutSuccessOrderNumber"><?php echo TEXT_YOUR_ORDER_NUMBER . $zv_orders_id; ?></div>
<?php if (DEFINE_CHECKOUT_SUCCESS_STATUS >= 1 and DEFINE_CHECKOUT_SUCCESS_STATUS <= 2) { ?>
<div id="checkoutSuccessMainContent" class="content">
<?php
/**
* require the html_defined text for checkout success
*/
require($define_page);
?>
</div>
<?php } ?>
<!-- bof payment-method-alerts -->
<?php
if (isset($_SESSION['payment_method_messages']) && $_SESSION['payment_method_messages'] != '') {
?>
<div class="content">
<?php echo $_SESSION['payment_method_messages']; ?>
</div>
<?php
}
?>
<!-- eof payment-method-alerts -->
<!--bof logoff-->
<div id="checkoutSuccessLogoff">
<?php
if (isset($_SESSION['customer_guest_id'])) {
echo TEXT_CHECKOUT_LOGOFF_GUEST;
} elseif (isset($_SESSION['customer_id'])) {
echo TEXT_CHECKOUT_LOGOFF_CUSTOMER;
}
?>
<div class="buttonRow forward"><a href="<?php echo zen_href_link(FILENAME_LOGOFF, '', 'SSL'); ?>"><?php echo zen_image_button(BUTTON_IMAGE_LOG_OFF , BUTTON_LOG_OFF_ALT); ?></a></div>
</div>
<!--eof logoff-->
<br class="clearBoth" />
<!--bof -product notifications box-->
<?php
/**
* The following creates a list of checkboxes for the customer to select if they wish to be included in product-notification
* announcements related to products they've just purchased.
**/
if ($flag_show_products_notification == true) {
?>
<fieldset id="csNotifications">
<legend><?php echo TEXT_NOTIFY_PRODUCTS; ?></legend>
<?php echo zen_draw_form('order', zen_href_link(FILENAME_CHECKOUT_SUCCESS, 'action=update', 'SSL')); ?>
<?php foreach ($notificationsArray as $notifications) { ?>
<?php echo zen_draw_checkbox_field('notify[]', $notifications['products_id'], true, 'id="notify-' . $notifications['counter'] . '"') ;?>
<label class="checkboxLabel" for="<?php echo 'notify-' . $notifications['counter']; ?>"><?php echo $notifications['products_name']; ?></label>
<br />
<?php } ?>
<div class="buttonRow forward"><?php echo zen_image_submit(BUTTON_IMAGE_UPDATE, BUTTON_UPDATE_ALT); ?></div>
</form>
</fieldset>
<?php
}
?>
<!--eof -product notifications box-->
<!--bof -product downloads module-->
<?php
if (DOWNLOAD_ENABLED == 'true') require($template->get_template_dir('tpl_modules_downloads.php',DIR_WS_TEMPLATE, $current_page_base,'templates'). '/tpl_modules_downloads.php');
?>
<!--eof -product downloads module-->
<div id="checkoutSuccessOrderLink"><?php echo TEXT_SEE_ORDERS;?></div>
<div id="checkoutSuccessContactLink"><?php echo TEXT_CONTACT_STORE_OWNER;?></div>
<h3 id="checkoutSuccessThanks" class="centeredContent"><?php echo TEXT_THANKS_FOR_SHOPPING; ?></h3>
</div>
<?php } ?>