1. Your "large font" is a result of your code changes outputting HTML in the wrong order, thus breaking the page layout and the browser getting confused.

2. The code you've been given (as quoted in your first post) is outdated. Neither of the code changes you posted is needed. Remove them and do the following instead:

Create a new file: /includes/modules/pages/checkout_success/jscript_idevaffiliate.php
Code:
<?php
  $idev_saleamt = number_format($order_summary['commissionable_order'], 2);
  $idev_ordernum = $order_summary['order_number'];
  echo "<script language=\"javascript\" type=\"text/javascript\" src=\"http://www.saffireblue.ca/idevaffiliate/sale.php?profile=13&idev_saleamt=$idev_saleamt&idev_ordernum=$idev_ordernum\"></script>";
Ref: http://www.zen-cart.com/content.php?...filiates-Tools