如何在我的购物车中保存两笔交易

Question

i have two transaction on my cart.

while i click checkout, why $grand total only one inserted to my db. and when i echo $grand, it appear two total.

this is my function

function cart(){
    $username = $_SESSION['UserID'];
    $cart = array();
    $sql = mysql_query("SELECT * FROM cart WHERE Username='$username'");

    while ($r=mysql_fetch_array($sql)) {
        $cart[] = $r;
    }
    return $cart;
}  

this is my process code

for ($i = 0; $i < $j; $i++){
    echo $start= $cart[$i]['City_Saler'];
    echo $end= $cart[$i]['City_Cust'];
    $price= $cart[$i]['Price'];
    $gram= $cart[$i]['gram'] / 1000;
    echo $kg= ceil($gram);

  $query= mysql_query("select * FROM service WHERE Start_Destination='$start' AND End_Destination='$end'");
  while($query_row = mysql_fetch_assoc($query)){

      $grand=$query_row['ServPriceKG'] + $price ;  

  }
} 

for ($i = 0; $i < $j; $i++){
  mysql_query("INSERT INTO ordering_detail(ID_Transaction, ID_Product, User_ID, Service_Name, Service_Package, Qty, gram, Price, Receiver_Name, Receiver_City, Receiver_Subdis, Receiver_Address, Receiver_Prov, Receiver_Phone, Grandtotal, Payment, Stat_Transfer_Item, ID_Saler)

  VALUES('$idtransaction', '{$cart[$i]['ID_Product']}', '$cusid', '$service', '$paket', '{$cart[$i]['qty']}', '$kg', '{$cart[$i]['Price']}', '$name', '$reci', '$subdis',  '$address', '$prov', '$phone',  '$grand', '-', '-', '{$cart[$i]['ID_Saler']}')");
}

      mysql_query("DELETE FROM preview_ordering WHERE ID_Preview= '$id'");


for ($i = 0; $i < $j; $i++) { mysql_query("DELETE FROM cart WHERE ID = {$cart[$i]['ID']}");}

header('Location:cart.php');

please help me out