I think that after 3 months she can't leave feedback... Would someone confirm that?
Get her to fill out the right form for lost mail (basically signing off on the fact that she didn't get it, with the threat of fraud charges if she is lying). If you sent it small packet with canada post then you have up to $100 CAN insurance.
However if you sent it just regular mail, then you have to decide.
I decided long ago that if I was saving on the postage and not paying for the insurance, replacing or refunding one or two a year was just going to be a business expense. I save on the postage, and part of that saving went to customer relations.
Good luck,
Buck