<?php

class SV_SandboxPaypal_NixFifty_GiftUpgrades_Model_GiftUpgrade extends XFCP_SV_SandboxPaypal_NixFifty_GiftUpgrades_Model_GiftUpgrade
{
    public function validateRequest(&$errorString)
    {
        if (!XenForo_Application::debugMode())
        {
            $errorString = new XenForo_Phrase('sandboxing_paypal_but_debug_is_off');
            XenForo_Error::logException(new Exception($errorString), false);
            return false;
        }

        if (empty($this->_filtered['test_ipn']))
        {
            $errorString = new XenForo_Phrase('sandboxing_paypal_but_live_paypal_in_use');
            XenForo_Error::logException(new Exception($errorString), false);
            return false;
        }

        return parent::validateRequest($errorString);
    }

    public function buildPayPalQueryUrl($paymentParams)
    {
        $queryString = XenForo_Link::buildQueryString($paymentParams);
        $url = "https://www.sandbox.paypal.com/cgi-bin/webscr?{$queryString}";

        return $url;
    }
}
<?php

class SV_SandboxPaypal_NixFifty_GiftUpgrades_Model_GiftUpgrade extends XFCP_SV_SandboxPaypal_NixFifty_GiftUpgrades_Model_GiftUpgrade
{
    public function validateRequest(&$errorString)
    {
        if (!XenForo_Application::debugMode())
        {
			$errorString = new XenForo_Phrase('sandboxing_paypal_but_debug_is_off');
			XenForo_Error::logException(new Exception($errorString), false);
			return false;
        }

        if (empty($this->_filtered['test_ipn']))
        {
			$errorString = new XenForo_Phrase('sandboxing_paypal_but_live_paypal_in_use');
			XenForo_Error::logException(new Exception($errorString), false);
			return false;
        }

        return parent::validateRequest($errorString);
    }
}
--- 
+++ 
@@ -6,18 +6,26 @@
     {
         if (!XenForo_Application::debugMode())
         {
-			$errorString = new XenForo_Phrase('sandboxing_paypal_but_debug_is_off');
-			XenForo_Error::logException(new Exception($errorString), false);
-			return false;
+            $errorString = new XenForo_Phrase('sandboxing_paypal_but_debug_is_off');
+            XenForo_Error::logException(new Exception($errorString), false);
+            return false;
         }
 
         if (empty($this->_filtered['test_ipn']))
         {
-			$errorString = new XenForo_Phrase('sandboxing_paypal_but_live_paypal_in_use');
-			XenForo_Error::logException(new Exception($errorString), false);
-			return false;
+            $errorString = new XenForo_Phrase('sandboxing_paypal_but_live_paypal_in_use');
+            XenForo_Error::logException(new Exception($errorString), false);
+            return false;
         }
 
         return parent::validateRequest($errorString);
     }
+
+    public function buildPayPalQueryUrl($paymentParams)
+    {
+        $queryString = XenForo_Link::buildQueryString($paymentParams);
+        $url = "https://www.sandbox.paypal.com/cgi-bin/webscr?{$queryString}";
+
+        return $url;
+    }
 }
Add support for Gift Upgrades 1.4.1
