yes, you need to loop through the cart items.
$eCart1->MoveFirst();
while (!$eCart1->EOF()) {
$FinalStack = $eCart1->DisplayInfo("subtractions");
$idProduct = $eCart1->DisplayInfo("ID");
mysql_select_db($database_connChan, $connChan);
$subtractions = mysql_query("UPDATE Lamps SET LampInventory = '$FinalStack' WHERE LampID = '$idProduct'") or die(mysql_error());
$eCart1->MoveNext();
}
$eCart1->MoveFirst();