Notwithstanding that I'm not a fan of the overall approach you're using with your site, and I'm not a fan of changing these links to no longer take the customer directly to their order, you should be able to accomplish what you requested by making the following changes in /includes/classes/order.php

1. for Text-Only emails:
Change this:
Code:
    EMAIL_TEXT_INVOICE_URL . ' ' . zen_href_link(FILENAME_ACCOUNT_HISTORY_INFO, 'order_id=' . $zf_insert_id, 'SSL', false) . "\n\n";
to this:
Code:
    EMAIL_TEXT_INVOICE_URL . ' <a href="http://bmisurgery.org/index.php?ID=supplements-new" title="BMI Surgery Store">BMI Surgery Store</a>' . "\n\n";
2. for HTML-formatted emails:
Change this:
Code:
    $html_msg['INTRO_URL_VALUE']       = zen_href_link(FILENAME_ACCOUNT_HISTORY_INFO, 'order_id=' . $zf_insert_id, 'SSL', false);
to this:
Code:
    $html_msg['INTRO_URL_VALUE']       = '<a href="http://bmisurgery.org/index.php?ID=supplements-new" title="BMI Surgery Store">BMI Surgery Store</a>';