Take a picture of the package and of the damaged article.
Send it to the buyer through eBay messages.
Since the item was returned the buyer is not invested in it. She just wants her money back. USPS does not offer any recompense on uninsured items. They want to be paid for insurance.
You might try Canada Post, since the parcel also passed through their hands. Phone, but don’t get your hopes up.
Were there any markings on the parcel that might indicate where the damage occurred? Usually the postal service will mark such packages, as part of their “best practice” to avoid fraudulent insurance claims.
You can try doing a partial refund on the damaged return. FWIW, and it is a little sleazy, the buyer cannot prove that she returned the item, since she did not use a Delivery Confirmed service. Which gives you some help with a Paypal/eBay claim, but as I said, is a little sleazy.
The buyer’s tools in this situation are feedback and Detailed Seller Ratings. Remember that DSRs that drop below 4.3 will lead to restrictions on your Seller account that can last for up to a year.