为什么没有正确的ID插入此表？

I want the following code to insert the car's ID in the customer_payment table, but it only selects 477 as the ID. I don't know why. As it can be seen in image bellow only product_id = 477 is inserted, not any other value. If I select 500 it still inserts 477.

include 'admin/db.php';

if(isset($_GET['payment_here'])){

    //select product id from cart
    $select_cart = "select * from cart";
    $runcart = mysqli_query($conn, $select_cart);
    $cartwhile=mysqli_fetch_assoc($runcart);

    $carssid = $cartwhile['P_ID'];
    $cusid = $cartwhile['C_ID'];

    //select id from cars
    $scars = "select * from cars where id=$carssid";
    $scarsrun = mysqli_query($conn, $scars);
    $showcars = mysqli_fetch_assoc($scarsrun);
    $carsdealer = $showcars['dealer'];


    //select customer id from customer  table
    //$selectcust = "select * from customer_register where id=$cusid";
    //insert data into customer payment table
    echo $insertpay = "insert into customer_payment 
    (Product_id, customer_id, dealer) 
    values ( $carssid," . $_SESSION['customer_id'] . ", '$carsdealer')";
    $run_inserts = mysqli_query($conn, $insertpay);
    /*
    if($run_inserts){
        echo "<script>window.location.href = 'checkout.php'</script>";
    }
    */
}
?>

Why is the correct ID not being inserting into this table?

What you are trying to do here

$select_cart = "select * from cart";
$runcart = mysqli_query($conn, $select_cart);
$cartwhile=mysqli_fetch_assoc($runcart); // here

is fetching the first entry from the 'cart' table which is always going to be same.

You can try something like this.

$c_id = $_SESSION['customer_id'];
$select_cart = "select * from cart where C_ID=$c_id";
$runcart = mysqli_query($conn, $select_cart);
$cartwhile=mysqli_fetch_assoc($runcart);

This query will specifically fetch data for customer of current session. The rest of the code you can use as is.