If the buyer is not in Canada you can't.
You will have to assure the buyer that you will reimburse his postage costs when you make his refund.
Let him know that this will be a separate payment using Paypal's Send Money.
Since you know the approximate weight and dimensions of the original parcel, look on his nation's PO website for an appropriate service-- tracked/Delivery Confirmed-- and tell him how much that service will cost.
Unless you are both very cordial about this return, do NOT send the money for the return postage until you have the item in hand.