Hi, I'm not sure if I can help but I'll try. We're the second pair sent with tracking and/or insurance?
I don't think I would send a third pair. To close the case, I would issue a refund. Unfortunately it is a loss but there seems to be something suspicious about this transaction. Be very professional and perhaps say something like "it's unfortunate the postal service is so flawed, but after two attempts I am at a loss".
I have asked about blocking a person in the forums before and was told it can be done, but the person was not sure how. I know Etsy has Seller Protection (in your private shop view, on the left side, under 'Resources'.
I hope this is of some help. Best of luck.