The fees kick in as soon as the item is sold. So if you do not get paid because for any number of reasons the transaction does not proceed, you will need to file an Unpaid case against the buyer, or offer a Mutual cancellation. The following link explains all about final value fees:
http://pages.ebay.ca/help/sell/fvf.html
You really ought to have included a shipping fee, - it's the wise thing to do. So use metric to weigh, measure, and enter the info at the Canada Post site, and inform your buyer of the cost.
To do a cancellation, the buyer needs to agree. It is possible that he could decline to accept, and then you would not get your fees back. Your email exchange will let you know how to handle it. With a cancellation, he can leave feedback. Here is a link to tell you more:
http://pages.ebay.ca/help/sell/cancel-transaction-process.html
The other alternative is the Unpaid Item dispute, - you get your fees back, and no one leaves feedback. But if you do that, and the buyer pays to avoid a strike, you run a good chance of getting a negative. After all, why would you opt for such a hostile act when it was you who neglected to mention a shipping fee. But this link will tell you how Unpaid cases work:
http://pages.ebay.ca/help/sell/unpaid-items.html
I'd say get the best price you can for shipping and contact the buyer with it. Who knows, he may be understanding and promptly pay it.
