it appears the "classic site" link has gone for good. i've never bothered adding tracking numbers for ebay-related purchases on paypal - i just add them to a dispute / case should the rare occasion happen where one is opened. have never had a problem doing it that way.
though, if you still want to use the "classic site" format - you can use this link - https://history.paypal.com/cgi-bin/webscr?cmd=_history
and you should be able to still access transactions in the old format and be able to click "Add Tracking Info"
hope that works! :]
