Having sent thousands of parcels by Canada Post, in over 30 years in retail and mail order, selling everything from wood stoves to postage, I can flatly say that I've had less than a dozen claims for items 'lost in the mail.'
Tell the scammer customer, politely, that you are sorry he is unhappy with the replacement notes. Tell him that when you receive them back you will send him a full refund.
If he does, fine. Refund him and put him on your Blocked Bidder List.
If he does not and makes any other demands. Repeat. Full refund on return. Full refund on return. Full refund on return. Put him on your Blocked Bidder List.
And to answer your question about responsibility, when you ship to your customer you are responsible for secure delivery.
You delegate shipping to Canada Post and as their customer, they are responsible to you. But unless you pay for an insured service, they will not be helpful. You remain responsible to your customer.
Politely.-- Captain Malcolm Reynolds