well the short answer is NO.........BUT......if your AMEX is drawn on a Canadian bank it works (sometimes).......on a US bank it does not......
I assume your in the States..........there is the "odd time" where you can go to a Western Union location and pay with a credit card.....but retailers are not supposed to do this......but I have seen it happen (at a grocery store for example)..........if you call Western Union from the states and ask, I'm 99% sure they will tell ya no.
hope that helps.....
so ummmmm....guess the answer is no