Well, it's been my experience that Celine's (and other agencies) don't really care if you are a citizen of the U.S., Canada, or Mars.
Paying in U.S. funds presents the agency with the extra burden of having to exchange it into CDN funds for their purposes, as well as trying to determine the best exchange rate for them to get. So for them to charge a "convenience fee" for using U.S. funds is understandable. And I agree that the best thing to do, time and opportunity allowing, is to exchange into CDN funds first, allowing an arbitrary 3rd party such as a bank to determine the exchange rate.
After all, would you even think about paying an SP in the U.S. with non-U.S. funds?
And even if this is still unacceptable to you, then you can always try other agencies, which may offer a reasonable exchange rate, take US cash at face value, or charge less of a "convenience fee" for using U.S. funds.