在快速结账时不能传递负值PayPal

在我尝试为优惠券折扣添加负值之前，我的PayPal快速结账效果很好。基本上，我无法理解如何传递负值。请在下面查看我的脚本：

//pass all products
foreach($_POST['item_name'] as $key=>$itmname)
{
    $product_code   = filter_var($_POST['item_code'][$key], FILTER_SANITIZE_STRING); 
    $results = $dbh->query("SELECT name, price FROM products WHERE id='$product_code' LIMIT 1");
    $obj = $results->fetch_object();
    $paypal_data .= '&L_PAYMENTREQUEST_0_NAME'.$key.'='.urlencode($obj->name).
                    '&L_PAYMENTREQUEST_0_NUMBER'.$key.'='.urlencode($_POST['item_code'][$key]).
                    '&L_PAYMENTREQUEST_0_AMT'.$key.'='.urlencode($obj->price).      
                    '&L_PAYMENTREQUEST_0_QTY'.$key.'='. urlencode($_POST['item_qty'][$key]);
    // item price X quantity
    $subtotal = ($obj->price*$_POST['item_qty'][$key]);
    //total price
    $ItemTotalPrice = $ItemTotalPrice + $subtotal;
}
##############################################
// Trying to pass a negative value in this way
if($_SESSION["Coupon_discount"] > $ItemTotalPrice){
        $cname = "Coupon Discount";
        $ccode = "code";
        $cqty = 1;
        $camt = 5;
    $paypal_data .= '&L_PAYMENTREQUEST_0_NAME='.urlencode($cname).
                    '&L_PAYMENTREQUEST_0_NUMBER='.urlencode($ccode).
                    '&L_PAYMENTREQUEST_0_QTY='.urlencode($cqty).
                    '&L_PAYMENTREQUEST_0_AMT='.urlencode($camt);        
}
##############################################

// Others Paypal process
$TotalTaxAmount     = 0; 
$HandalingCost  = 0;
$InsuranceCost  = 0;
$ShippinDiscount    = 0; 
$ShippinCost        = 0;
$GrandTotal = ($ItemTotalPrice + $TotalTaxAmount + $HandalingCost + $InsuranceCost + $ShippinCost + $ShippinDiscount);
$padata =   '&METHOD=SetExpressCheckout'.
            '&RETURNURL='.urlencode($PayPalReturnURL ).
            '&CANCELURL='.urlencode($PayPalCancelURL).
            '&PAYMENTREQUEST_0_PAYMENTACTION='.urlencode("SALE").
            $paypal_data.               
            '&NOSHIPPING=0'. 
            '&PAYMENTREQUEST_0_ITEMAMT='.urlencode($ItemTotalPrice).
            '&PAYMENTREQUEST_0_TAXAMT='.urlencode($TotalTaxAmount).
            '&PAYMENTREQUEST_0_SHIPPINGAMT='.urlencode($ShippinCost).
            '&PAYMENTREQUEST_0_HANDLINGAMT='.urlencode($HandalingCost).
            '&PAYMENTREQUEST_0_SHIPDISCAMT='.urlencode($ShippinDiscount).
            '&PAYMENTREQUEST_0_INSURANCEAMT='.urlencode($InsuranceCost).
            '&PAYMENTREQUEST_0_AMT='.urlencode($GrandTotal).
            '&PAYMENTREQUEST_0_CURRENCYCODE='.urlencode($PayPalCurrencyCode).
            '&LOCALECODE=GB'.
            '&LOGOIMG=http://www.myweb.com/images/logo.png'. 
            '&CARTBORDERCOLOR=000000'. 
            '&ALLOWNOTE=1';
    // SetExpressCheckOut method to obtain paypal token
    $paypal= new MyPayPal();
    $httpParsedResponseAr = $paypal->PPHttpPost('SetExpressCheckout', $padata, $PayPalApiUsername, $PayPalApiPassword, $PayPalApiSignature, $PayPalMode);
    // Success or Error done here

这里的问题星在添加这些行后

    if($_SESSION["Coupon_discount"] > $ItemTotalPrice){
        $cname = "Coupon Discount";
        $ccode = "code";
        $cqty = 1;
        $camt = 5;
    $paypal_data .= '&L_PAYMENTREQUEST_0_NAME='.urlencode($cname).
                    '&L_PAYMENTREQUEST_0_NUMBER='.urlencode($ccode).
                    '&L_PAYMENTREQUEST_0_QTY='.urlencode($cqty).
                    '&L_PAYMENTREQUEST_0_AMT='.urlencode($camt);        
}