public static function payment($amount, $localTrxID = null) {
		$impl = new PayPal();
		
		$registration = $impl->registerTransaction($amount, $impl->storeLocalTrx($localTrxID));
		Session::instance()->set(self::SESSION_TOKEN, $registration->id);
		
		foreach ($registration->links as $link) {
			if ($link->rel == 'approval_url') {
				HTTP::redirect($link->href);
				exit; // shouldn't be needed, redirect throws
			}
		}
		throw new PayPal_Exception_InvalidResponse('Missing approval URL');
	}