I'm echoing another post - I had carefully acquired a chip & pin MasterCard from USAA for this trip since I am renting a car. So far all the toll machines we have seen have NOT allowed MasterCard and most have not allowed Visa. So cash is king! We have been on the A10, A77, and A85; I'm not sure if this is universal or if we have just been unlucky.
Thanks Bets for the info on Aires - we have stopped at several so far. And thanks to everyone whose advice was so helpful during the lengthy planning process. I'll be posting reviews and a trip report after we return but wanted to post the info on the toll payment options (or lack thereof).