This is only an issue for people using paypal standard and IPN, and it has already been addressed in eCart and the latest version of Powerstore doesn't support Paypal standard since Express Checkout only is available in its place.
This shouldn't be a concern for most customers. If you are using IPN and think this does apply to you, then you should just have to update the: WA_eCart/WA_eCart_Definition_PHP.php to the latest version and it will be fixed.