In the short run, send the buyer a Paypal 'send money' invoice. Let him know this is coming and that he will still be covered by both eBay and PP Buyer Protection.
Then look again at your other listings, from eBaydotCOM.
Check the shipping. The most likely problem is that your dimensions and weight for Calculated Shipping to the USA do not match the USPS criteria, which are a little different from Canada Post.