Have the seller start a claim.
Canada post... Don't get me wrong, I'm a supremely mellow guy but After months of shitty service, missing mail, and bungled parcel delivery like you describe, I "went postal" at the local post office and gave the person there a strongly worded piece of my mind. Then I ambushed my local carrier and the parcel person and did the same. Then I called CP to complain.
What followed was a solid 18 months of impossibly bad service. Complaints made it worse, far worse than I thought possible. At the end they didn't even attempt to deliver my parcels anymore, just threw the notices on my doorstep - usually long after the parcel had been ready for pickup.
Then I moved. Now I tip my carrier and parcel guy at Christmas, and go out of my way to chat up the local PO ladies and the service has been flawless.