You are on eBaydotCA?
Okay, that happens from time to time, usually in new suburbs when the Canada Post and eBay systems have not 'synced'.
The purpose of the postal code is identifying where the items are located, which can make a difference in both Calculated Shipping and in your future customer's import fees.
So.
Find out the postal code for your municipal building/City Hall and use that.
Did it help?