My company is using OSC version 2.3.4 with paypal express checkout for a few years due to the easy setup. But recently we need to use the discount coupon with paypal, which we cannot pass the value via paypal express checkout, so we want to change to use paypal standard for paypal payment. But our client reported that they can pay via Paypal after switching to paypal standard 3.1 (by default), and we received the money, but it appeared that the result is not return back to our system and it does not update the stock level, the client shopping cart and the order status. We want to update the paypal app 5.0 from the appstore, but yet it is saying that OSCOM_PayPal.php is missing.