I had the same problem. Canada Post is always adding new postal codes however, I don't think Ebay does much to update their end in retrieving the most up to date information from Canada Post which would include all the newly added postal codes. My residence was issued a new postal code and I ended up getting the same error. I got around this by using the initial older assigned postal code which was recognized. If you postal code is newly assigned, it may be that it's simply not recognized by ebay who may be relying on old data.
