Hmm, it would appear that, where I thought this had been fixed previously (in 2012
SD discussing it being fixed in a release, which coudln't have been 7.6 since he wasn't a part of the project, and its recent), it would appear that the sub_thanks.inc.php and sub_cancel.inc.php files still haven't been supplied with a release (when they should have been included with the base PayPal coding added in v7.3 by Rick).
Isaac and I spent some time today with the PayPal API code and testing newer coding for a future release.