Is the seller in Canada?
If so she can send you a Canada Post mailing label.
If she is NOT in Canada, she cannot send you a mailing label.She can't.
It is not possible.
There is no way to do this.
The seller seems to have agreed to refund you when the item arrives back. You will have to use a little trust and send it back on your own dime to receive any refund at all.
If you don't send it back, she is not required to refund and Paypal /eBay will back her on that.
And again:
If she is NOT in Canada, she cannot send you a mailing label.
EBay phone reps tend to have trouble with concepts like 'Canada'.
The seller, when she gets the item back can refund you from her Paypal account for the original payment. This takes seconds. She will have to send you a separate payment for your return parcel, which takes minutes.
If she is NOT in Canada, she cannot send you a mailing label.