I would try changing:
$ExecuteQuery->Statement = "UPDATE customer_orders SET customer_orders.OrderTotals = customer_orders.OrderTotals + $OnlineOrders->getColumnVal('OrderSubTotal') WHERE customer_orders.Email = $OnlineOrders->getColumnVal('OrderEmail')";
$ExecuteQuery->execute();
to:
$ExecuteQuery->Statement = "UPDATE customer_orders SET customer_orders.OrderTotals = customer_orders.OrderTotals + " . $OnlineOrders->getColumnVal('OrderSubTotal') ." WHERE customer_orders.Email = '". $OnlineOrders->getColumnVal('OrderEmail') ."'";
$ExecuteQuery->execute();