On Thursday I charged CAD$34.25 to my Amex. Today (Sunday) it posted as US$28.29. Oanda.com says that at today's interbank rate it = $27.73, at Thu.'s interbank = $27.62. Sounds like interbank + 2%.

Remember that Visa or MC and the Issuing Bank EACH get a cut, while Amex issues its own cards directly, so only one cut . However, few European merchants accept Amex.