First you contact the seller and ask for a refund, explaining the problem.
If she agrees, you go here:
https://www.canadapost.ca/cpotools/apps/far/business/findARate?execution=e1s1
and enter the weight and measurements of the (re-wrapped) parcel.
Choose a service that will give you Confirmation of Delivery (sometimes called tracking).
You can then go here:
https://www.canadapost.ca/personal/tools/cst/intro-e.asp
and print a shipping label using your credit card for payment.
OR you can take the parcel to your PO and buy a label at the counter.
Give the labelled parcel to the clerk. Get your stamped receipt.
Tell the seller the 'tracking' number on the parcel. Keep your receipt with the 'tracking' number.
When the parcel gets back to the seller, she is supposed to refund you promptly.
If that doesn't happen, or if you have already been refused by the seller, come back here and let us know.
You have three other ways to proceed.