public function GetPaymentURL($orderId, $amount, $title = null, $customData = null)
{
$config = $this->_getProviderInfo();
if ($customData) {
$customData = json_encode($customData);
}
$paypal = new paypal_class();
$paypal->add_field("cmd", "_xclick");
$paypal->add_field("business", $config['account']);
$paypal->add_field("return", $this->returnURL);
$paypal->add_field("cancel_return", $this->cancelURL);
$paypal->add_field("quantity", 1);
$paypal->add_field("amount", $amount);
$paypal->add_field("item_name", $title);
$paypal->add_field("item_number", $orderId);
$paypal->add_field("undefined_quantity", 0);
$paypal->add_field("no_shipping", 1);
$paypal->add_field("no_note", 1);
$paypal->add_field("rm", 2);
$paypal->add_field("custom", $customData);
$paypal->add_field("charset", 'utf-8');
$paypal->add_field("currency_code", $this->currencyCode);
$url = $paypal->build_param_url();
return $url;
}