I believe any fees are generated by your own bank, not the Mexican bank the transaction is going through. So, the best course of action would be to check with your bank to see what kind of international transaction fees they charge if any. I occassionally use my US card down here and there don't seem to be any extra fees on my statement, other than sometimes a not super favourable exchange rate.
A couple of heads up though. Mexico does have a reputation for credit card fraud so many banks will automatically blocked CCs once used down here. Make SURE that your bank is aware you will be travelling to Mexico ahead of your trip. I also try never to use my card at any really small shops or anywhere where my card will be out of my sight while they process it. Keep cash on hand for small stuff.
Hope that helps!!
Lori's Office Assistant