Disabling An Exit Popup Script When Submitting Form?

We implemented an exit popup on our page. When a user tries to leave our purchase page, an exit popup comes up saying that if they click cancel they will get a 50 dollar discount and are automatically redirected to that discounted checkout form.

The problem is, when a user clicks our "submit" button, when they are trying to purchase at full price, the exit popup shows up too, and redirects them, so essentially we can't get the full amount out of our customers! We want the script on the page, but we don't want it to show up when users click our submit button.

heres the exit popup script

Code:
<script language="JavaScript" type="text/javascript">
window.onbeforeunload = confirmExit;
function yPop(url) {
var found = 1;

[Code].....