you would need to hand code this in the Update Cart server behavior that is added to the page.
the update cart behavior would look similar to:
<?php
// WA eCart Update
if (isset($_POST["eCart3_Update_100"]) || isset($_POST["eCart3_Update_100_x"])) {
$eCart3->UpdateCart();
$Redirect_redirStr="";
if ($Redirect_redirStr != "") {
$eCart3->redirStr = $Redirect_redirStr;
}
$eCart3->cartAction = "Update";
}
?>