You want to pay for your buyer to return an item?
Unless he is also in Canada, you will need to go to a courier that ships internationally like UPS.
If he is in Canada, you can go to canadapost.ca and set up a shipping label from his address to yours. Make sure you know the dimensions and weight of the parcel he is returning. Then email a copy of the label to him.
He prints it out - so make sure he has a printer before you pay for it- tapes it onto the parcel and drops it in the nearest postbox.