Couple of options to avoid exchange / service costs when up here in Canada'
I, too had an AMEX card and it was stolen while in Italy. Got it cancelled and when back, ordered a new one. They wanted a full year replacement cost even though 1/2 of the year had elapsed . see ya said I !! As mentioned by others, AMEX wants money for anything and they have the highest processing fees for Merchants.
Check with you card head office re your question re out of country use. regardless of your card, you will find all of them can be used up North with no extra fees. Your statement might state 35.00 @ ( the daily rate of exchange) and then that cost is carried to be your cost in US dollars. so you should gain at least 20% when shopping in Canada.
I have a VISA card which is in US dollars should I go South and I pay in Cdn. dollars plus exchange rate when invoice is due. VISA also has EUROs and English Pounds if you ever travel over seas. This might be just a Canadian thing but ask your bank/credit union.
You might want to consider applying for a Canadian dollar Visa or Mastercard from a Canadian bank / credit union but your mailing address would be in the USA. That way you pay the bill in US dollars but getting the exchange rate is a bonus.
There might be some regulations which prevent this happening but go online and search out the major banks in Canada and pose the question-you might get lucky.
There are rules and regulations when coming in to Canada at the border-food, vegetables and fruit, firearms ( not possible as we have very different rules re any sort of firearms. bear bangers etc. most stores and hotels also have the Interac option as well.
Contact the closest Canadian Embassy or consulate if you have questions- remember, there is no such thing as a silly question-only a silly answer.
enjoy the vistt