Thank you all for your wonderful advice!
I wish I could say that USPS was of any help, but in the 3 times that I called them, every time they told me there was nothing I could do because I sent it First Class and they aren't even required to scan the package in at any point along its journey. Canada isn't required to either. So, the customs number tracking info says that it was processed at the Philadelphia sorting facility on November 7th and that's it. It doesn't even say that it departed the facility, but I was explained that it doesn't mean it didn't leave the US. When you ship First Class International, it's basically up to the person processing it if they feel like scanning it in that day. Canada Post has no info on the package either. All of the prints are completely custom, so I wouldn't be able to resell them if I shipped her new ones and she ended up getting the original package. And, when I asked USPS about claiming my insurance on the package, they told me that I can submit a claim, but it would never be approved because it is First Class and there's no way of proving that it is actually lost.
I do explain in my policies that I ship First Class International and that it does not include tracking info, but as much as I would like to tell the customer that it is not my fault, I can't bring myself to purposefully upset her. I have a feeling I am just going to have to take a loss on this one. And the worst part is, her friend placed the same order (9 custom prints to Canada) a week later and she has not received them either. So, I might have to do this twice!