You can do it with Shippo. goshippo.com
Sign up or sign in. In the top right corner there will be a button that says 'Create Label'.
You may need to ask the buyer for dimensions. You might need the buyer's information (phone number, e-mail) for customs. If you go to your sold items, and click 'View Order Details' from the drop down menu to the left of this item, that information will be available. The e-mail will be an eBay e-mail that redirects to the buyer, but for these purposes that should be fine.
After you create the label, you can download it as a PDF. You can then send the buyer the PDF as an attachment via e-mail. Or, you can use Windows Snipping tool to take a screen shot of the PDF and turn it into a JPG, which can be attached directly in the eBay case. If you do e-mail the return label be sure to provide some sort of proof in the case that you've sent it.
If the cost of a label is high (both the price, and the time spent by you) relative to the cost of the item. You also have the option to try and negotiate something other than a return, like a partial refund. If something is my fault, or I feel the buyer is acting in good faith, I'm generally pretty open to that. When the buyer seems to be operating in bad faith, I'd rather take a loss and make them return the item. While it is rare, some buyers know how much of a hassle US to Canada returns are, so they try to leverage that for a partial discount on an item that is otherwise without issue.
