Copy of store invoice not sending when product is digital

My client is telling me that she fails to receive a copy of the Store Invoice when a digital (download) product is purchased via PayPal -- however, she does receive a PayPal notification and the invoice appears in 'Customers' > 'Orders' and is processed correctly -- she just doesn't get an email about it (and doesn't want to log in to the backend).

However, if a physical product is purchased, she receives both the Store Invoice and PayPal Notification.

Is there any reason why a digital product purchase's workflow is working differently than a physical one? I don't see separate settings in Email Options. I've enabled archiving for the future to see if the emails are being sent, but I figured I'd ask here as well.

The cart is running 1.3.5

From my own testing, adding this code around line 173 to /includes/auto_loaders/paypal_ipn.core.php has solved the issue:

/**
 * Breakpoint 130.
 * 
 * messageStack = new messageStack();
 * 
 */
  $autoLoadConfig[130][] = array('autoType'=>'classInstantiate',
                                 'className'=>'messageStack',
                                 'objectName'=>'messageStack');

The cart is running 1.3.5
Lots of bugs in that version..suggest upgrading
to latest zencart release.

To clarify, is it possible that Email Confirmation could work for some orders, but not for others? In my line of thinking, they'd either all work or wouldn't work. I only use PayPal to process, so it's not an issue of different Payment methods.