seams to me from the page you sent that it is the payment page that is changing the domain:
<form class="eC_Simple_Pacifica_Arial" action="https://pcisecure.diamondmindschools.com/cart/cart.php" method="POST">
Donation Total: <input type="text" size="8" name="amount" value= <?php echo WA_eCart_DisplayMoney($ccds, $ccds->GrandTotal()); ?> />
by posting to the cart.php page.