How could the buyer open a Not Received dispute when they clearly did receive the tickets?  Sorry that was a rhetorical question...they clearly did.  You just need to hope that paypal does not "kindly" alter it to Not As Described.

Yes, you clearly do need to contact paypal, but at least you should not have to wait for the buyer to send the tickets back if they say they did not  receive them.  Good luck with it all...I would not like to be in your shoes.