First, cooperate with the claim.
Put the tracking number , if you have one into the claim.
Do NOT assume that eBay has it.
They don't.
Without your active cooperation, eBay(or more exactly a bunch of robots) will assume that you never sent the item at all.
With the tracking number, and many crossed fingers and toes, the Claim will be kicked upstairs for further consideration.
They are allowing the cases to be opened to keep the buyers happy.
Now we just have to hope that when they see tracking from Canada Post the promises made about delayed shipments will be kept.
And that will keep the sellers happy.
Probably stereotyping here, but I suspect part of the problem may be the mindset of your typical buyer, which is very likely different from that of mine. My customers tend to be older, and to actively enjoy reading and following instructions. Does that fit what you know of horror movie aficianados?
Best wishes.
Put the tracking number into every Claim.