Since you already know the weight and dimensions of the item, you can use this to determine the cost of shipping.
https://postcalc.usps.com/?country=10440
You want Shape and Size and scroll down on the next page to First Class International Parcel to get the least expensive TRACKED service.
Your postal clerk is right, she can't sell you any USPS services but you can send a USPS Return Shipping Label by signing in to Shippo (you will need a Canada Post Small Business Solutions number, but you probably already have that for the occasional discounts from Shippo, Paypal, or SnapShip).
From there you can choose several shippers including USPS.
Make a shipping label with the buyer as Sender and you as Receiver. Send the label as an attachment to an eBay message.
The buyer prints it out and ships.
I've only done this once. It worked even better than I had hoped because the buyer did not use the label, I won the Dispute, and kept the payment. Eventually I was repaid for the label.
I suspect that while most people are honest, a few, realizing that non-US sellers would not know how to send return shipping, will demand a return for refund, planning to keep both.
While you may prefer to deal with the problem without getting eBay involved, if you suspect shady dealings it is a good idea to have a Dispute in place and have the problem and its resolution on the record.
