The code you pasted looks correct. I would only make a couple of small adjustments to account for a) the grand total being 0 and b) the session variable not being set:
<?php
if (!session_id()) session_start();
if($ShoeCart->GrandTotal() > 0) {
$_SESSION["Amount"] = "".($ShoeCart->GrandTotal() * 100) ."";
}
?>
<input name="Amount" type="hidden" id="Amount" value="<?php echo(isset($_SESSION['Amount'])?$_SESSION['Amount']:""); ?>" />