Not at my bank, it won't cost any extra money, even if it's made out in CDN dollars (that's one of the reasons I chose my particular bank). But some banks do charge if it's in CDN dollars, and sometimes the fee is quite hefty. If it's in US dollars (which you should insist on anyway) I believe that you shouldn't have any problem anywhere, with no charge for cashing.
You should (in my opinion) always ask for an international money order made out in US dollars. That way you don't have to pay any exchange rate either.
I don't
think you can take a CDN postal money order to our post offices here and get them cashed like you can a US postal money order, but I've never tried it, so I'm not sure about that. It's not our postal system, it's theirs, so I don't think USPS will cash them.
Hope that helps, that's about the best I can do without knowing more :)
Diane
Contact me about how to get free shipping for postcards you buy!