First Block her from future transactions.
Go to the original listing. Click on Sell Similar. Click on Edit. Now you will be a the first page of the listing.
Scroll down to Shipping and see what you entered. (As opposed to what you more recently measured and entered into Canada Post).You may find a difference. If not:
List the item* (yes this may cost you a few cents-- change the title to your ebay ID and any options like subtitle to reduce this cost) Then look at the cost of shipping in the indexing new listing. Since you two share most of a postal code, this should show exactly the same as the original shipping.
You could also look at similar items
But basically you are being played by a bully.
Give her the amount you worked for an hour to find and if she continues to complain start an Unpaid Item Dispute. This has the advantage that if she doesn't pay, she cannot leave the negative feedback that she probably will, no matter what you do, because she can and she is a bully.
Block her.
* While you are fairly safe with the title change against someone finding and buying the item, you want to end the listing as soon as the research is done. If you can do a screenshot for her, so much the better. So all this is costing you money and time and stress. Block her.
