You said your item would be quite flat. Would it be flat enough that your package would be less than 2cm thick? If so, then you can send by oversize lettermail within Canada, which would cost you $1.28 to about $ 2.10 depending on the weight. Check this find a rate link.
http://canadapost.ca/cpotools/apps/far/personal/findARate?execution=e1s2If your package is over 2cm thick, then yes, your cost will be higher, but it shouldn't be that much cheaper to the US. A little cheaper, but not that much.