That's a common problem, usually caused when a seller uses calculated shipping for expedited USA. There have quite a few threads lately about it so I'll just copy and paste the info from another thread. If you want to read the whole thread you can see it here http://community.ebay.ca/t5/Seller-Central/Invalid-Postal-Zip-Code-Can-not-ship-to-address-Buyer-can...
One big issue is with Expedited Parcel Canada and USA. EP Canada has a minimum dimensions of 10 x 7 x 0.1 cm while EP USA has a minimum dimensions of 21 x 14 x 0.5 cm.
So if an item is 20x10x5 cm and you entered those dimensions, the Sell Your Item listing form will accept it since it is okay for the domestic EP Canada shipping option but no error message for EP USA as your USA International shipping option. You submit the listing and think all is okay. You check the listing and shipping options and enter Canada with a Postal Code and it works. Do the same test and change the country to USA and a zip code and it shows a shipping error message. Why? Because the dimensions are too small for EP USA.
The Sell Your Item form does little or no checking for dimensions or weight. When the listing goes live it does check for valid data and gives cryptic messages saying that it can't be calculated.
If you go to the Canada Post website Find A Rate web page, it also does little or no checking on dimension and weights also. So it might give a rate for something not meeting the minimum dimensions too.
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