Well, they're booking a rate for you, but you're still paying the rental company at time of rental, not Costco. You could book that rate, then convert money and keep it in your USD account and use a USD CC if you wanted to be sure of your rate.
Anyway, I didn't know that you could cancel Expedia. Interesting. Do they charge you upfront and then refund you or is it the same... just a booking but a guaranteed price that you pay in the end? Do you pay Expedia or the rental company?