Cross-border?
Not possible*.
And eBay has yet to realize this.
So.
It comes down to a matter of trust between you and the seller. (You are the buyer, right? A lot of posters seem to have trouble with that.)
Tell your seller the cost of a (Canada Post) shipping label. Use a service that includes Confirmation of Delivery/ tracking.
https://www.canadapost.ca/cpotools/apps/far/business/findARate?execution=e1s1
Tracked packet, Expedited and ExpressPost are all tracked.
Choose the cheapest, they are all equally fast and reliable. The differences are not important to you.
Then, having discussed the problem with the seller, so she is not shocked and horrified by the cost of return shipping, buy the label and ship.
You can get a small discount if you happen to have a Small Business discount card. Ask about this at the Post OFFICE counter. But it really is a very small amount.
Your seller may prefer to accept photos of a damaged item over the high cost of return shipping.
I used an imaginary object weighing just under a pound and the size of a Stephen King paperback, and came out with rates in the $15 + range.
Include the link in your message to her.
When your seller gets the item back, she can refund your original payment directly from Paypal, but she will have to make a separate refund for the return postage from Paypal's Send Money service.
Which is where the trust comes in.
Keep your communication polite and friendly. You are not enemies, you are just having a discussion about a problem that can be solved with goodwill.
If you are the seller (whatever happened to proofreading?), it's the same story but in reverse.
The USPS link is:
http://ircalc.usps.com/?country=10054
Tell the buyer to hit the arrow on the right to find the reasonably priced First Class International Parcel service which is tracked and has Confirmation of Delivery.
In either case, the returned parcel should be marked RETURNED MERCHANDISE to prevent problems with customs. Including a copy of the original invoice would probably help.
*Seriously. The Universal Postal Union and international postal systems just don't allow for it.
