In hindsight, you should have explained the cost of reshipment to the seller and come to an understanding of how much he was willing to pay of the return.
At this point, since he is probably in shock at the actual cost, I would suggest giving him a link to the applicable part of canadapost.ca so he can see that you are not yanking his chain.
Then suggest splitting the dirrerence.
Keep in mind, that while reimbursing return postage is a Best Practice, especially if the error was the seller's, the Standard Practice follows Paypal's policy which is that the buyer covers return postage.
Keep it friendly. Angry words will not be helpful.
Politely. --- Captain Mal Reynolds