add a named anchor just before the add to cart button form and change the add to cart button forms action to use the named anchor:
<a name="button1" id="button1"></a>
<form name="eCart2_1_ATC_0" method="POST" action="<?php echo $_SERVER["PHP_SELF"]; ?>#button1<?php echo (isset($_SERVER["QUERY_STRING"]) && $_SERVER["QUERY_STRING"] != "")?"?".$_SERVER["QUERY_STRING"]:""; ?>">