PHP Paypal express 10413 - php

Question

I'm using Paypal Express Checkout on my website and i receive this error:
SetExpressCheckout API call failed. Detailed Error Message: The totals of the cart item amounts do not match order amounts.Short Error Message: Transaction refused because of an invalid argument. See additional error messages for details.Error Code: 10413Error Severity Code: Error
When i enter a quantity of 1 everything is working fine but when i enter 2 or more i get the error.
I'm using SMARTY Templating.
<form class="pull-right" action="../Checkout/paypal_ec_redirect.php" method="POST">
<input type="hidden" name="L_PAYMENTREQUEST_0_NAME0" value="{$product_naam}"></input>
<input type="hidden" name="L_PAYMENTREQUEST_0_AMT0" value="{$product_productprijs}"></input>
<input type="hidden" name="L_PAYMENTREQUEST_0_QTY0" value="{$product_aantal}"></input>
<input type="hidden" name="PAYMENTREQUEST_0_ITEMAMT" value="{$product_subtotaal}"></input>
<input type="hidden" name="PAYMENTREQUEST_0_TAXAMT" value="{$product_btw}"></input>
<input type="hidden" name="PAYMENTREQUEST_0_AMT" value="{$product_totaalprijs}"></input>
<input type="hidden" name="currencyCodeType" value="EUR"></input>
<input type="hidden" name="paymentType" value="Sale"></input>
<input type="image" src="https://www.paypalobjects.com/webstatic/en_US/i/buttons/checkout-logo-large.png" alt="Check out with PayPal"></input>
</form>
</div>
</div>
</div>
Here is the PHP:
foreach ($getproduct as $row) {
$productaantal = $aantalbesteld;
$productnaam = $row['naam'];
$productprijs = $row['prijs'];
$btwtarief = $row['btw_tarief'];
$btwbedrag = ($productprijs*$aantalbesteld)/100*$btwtarief;
$subtotaal = ($productprijs*$aantalbesteld)-$btwbedrag;
$totaalprijs = $productprijs*$aantalbesteld;
}
//Product
$smarty->assign('product_naam', $productnaam);
$smarty->assign('product_aantal', $productaantal);
$smarty->assign('product_prijs', money_format('%.2n', $productprijs));
$smarty->assign('product_subtotaal', money_format('%.2n', $subtotaal));
$smarty->assign('product_btw', money_format('%.2n', $btwbedrag));
$smarty->assign('product_totaalprijs', money_format('%.2n', $totaalprijs));
$smarty->assign('product_bestelknop', 'Bestellen');
EDIT: Even this isn't working:
<form class="pull-right" action="../Checkout/paypal_ec_redirect.php" method="POST">
<input type="hidden" name="L_PAYMENTREQUEST_0_NAME0" value="{$product_naam}"></input>
<input type="hidden" name="L_PAYMENTREQUEST_0_AMT0" value="5.00"></input>
<input type="hidden" name="L_PAYMENTREQUEST_0_QTY0" value="2"></input>
<input type="hidden" name="PAYMENTREQUEST_0_ITEMAMT" value="10.00"></input>
<input type="hidden" name="PAYMENTREQUEST_0_TAXAMT" value="2.10"></input>
<input type="hidden" name="PAYMENTREQUEST_0_AMT" value="12.10"></input>
<input type="hidden" name="currencyCodeType" value="EUR"></input>
<input type="hidden" name="paymentType" value="Sale"></input>
<input type="image" src="https://www.paypalobjects.com/webstatic/en_US/i/buttons/checkout-logo-large.png" alt="Check out with PayPal"></input>
</form>

Answer 1

It says when they multiply quantity and amount it doesn't add up to what you specify as totalamount
$btwbedrag = ($productprijs*$aantalbesteld)/100*$btwtarief;
$subtotaal = ($productprijs*$aantalbesteld)-$btwbedrag;
$totaalprijs = $productprijs*$aantalbesteld;
Your VAT is calculated by taking 1% of the total price amount multiply by VAT rate. This assumes your product amounts are exclusive VAT.
Then in the next line you remove VAT from $subtotaal, why? It wasn't included in the first place. Or if it was then you should divide by 121 instead of 100.
$totaalprijs seems to be calculated assuming VAT is included already.
Did you check the values of your hidden fields in the source of your page?

Answer 2

When I changed the qty to 2, this is the request that is sent to PayPal.
paymentrequest_0_currencycode "EUR"
paymentrequest_0_amt "89.90"
paymentrequest_0_itemamt "71.02"
paymentrequest_0_taxamt "18.88"
paymentrequest_0_paymentaction "Sale"
paymentrequest_0_name "Sweaters van Superdry"
paymentrequest_0_qty "2"
Ideally, the request should be like this:
paymentrequest_0_itemamt=71.02
paymentrequest_0_taxamt=18.88
paymentrequest_0_amt=89.90
L_PAYMENTREQUEST_0_NAME0 = Sweaters van Superdry
L_PAYMENTREQUEST_0_AMT0= 35.51
L_PAYMENTREQUEST_0_QTY0= 2
This is causing the problem.
There is no paymentrequest_0_qty parameter.
You can test it here :
https://api-3t.sandbox.paypal.com/nvp?&user=us-30_api1.cri.com&pwd=EYFNSNUSV85CT34Z&signature=AH57zE.nAaElaFFAysViNA9TIte1AxtSpBjx2HLqHJOiu2js3l1Kd48i&version=70.0&METHOD=SetExpressCheckout&RETURNURL=http://www.paypal.com/test.php&CANCELURL=http://www.paypal.com/test.php&PAYMENTACTION=Sale&paymentrequest_0_itemamt=71.02&paymentrequest_0_taxamt=18.88&paymentrequest_0_amt=89.90&L_PAYMENTREQUEST_0_NAME0=Sweateranuperdry&L_PAYMENTREQUEST_0_AMT0=35.51&L_PAYMENTREQUEST_0_QTY0=2
Just click/copy-pasate the link above and see the reponse