Paypal Standard payments and issue with alterMailParams hook

I'm not sure if this is a bug but here goes.

I have installed the civicrm_pdfreceipts module that was developed by PeaceWorks with my Civicrm install.

Drupal - 6.2
Civicrm - 3.3

The issue i'm having is with CiviContribute. When making a paypal payment (Paypal Standard), the transaction goes through with no problem, but the receipt that gets sent is the incorrect one. The civicrm_pdfreceipts module uses the CRM_Utils_Hook::alterMailParams( $mailParams ); to add a customized pdf receipt to the mail attachment and alter the message text before sending.

When using the dummy payment processor the hook gets called and I can receive the correct receipt from the civicrm_pdfreceipts module. However, when using Paypal Standard it seems like the code to call the hook doesn't get executed and as a result a receipt from the Message Templates gets sent rather than the one from the civicrm_pdfreceipts module.

I've checked and tried to debug with no success.
I've looked in civicrm/CRM/Mailing/BAO/Mailing.php and civicrm/CRM/Utils/Mail.php.

Could there be another place where an email gets sent?

I've sent many live payments to test because using the Paypal Sandbox doesn't send out any mail messages. If there is a way to send an email from CiviContribute after a transaction completes with the Paypal Sanbox that would really help me out to debug.

Any help would be greatly appreciated.

Thanks.