I just had a look at your sale page for the 35mm PC Nikkor and there don't look to be any issues regardng selling to a buyer in Japan. I wouldn't have used insurance optional since of it gets lost you would likely be held accountable as sender. I trust there are no issues regarding the shipping methods and amount paid since you used the calculator in your sale.
Your sale ended on Friday and it's only Monday. There was nothing that I could see in your sale terms about contact after the sale payment.
Again, your choices are to send to the Paypal address as per the shipping and insurance paid for by the buyer or wait for another 4-5 working days. But, barring any unresolved issues, and presuming you can ship insured by Global Priority or Express, I would send it off. If you must cancel the sale you will need to return the money sent to you via a Paypal transaction to the buyer. The buyer's account is part of the details. You would be within your rights, I believe, to deduct any Paypal and eBay costs incurred by you, but no more and you should document this precisely in the correspondence (or message area in Paypal's transaction) with the buyer.
Good luck