This is an FYI for anyone renting a car in Portugal.
Just got back from trip to Portugal. We rented a car (Europcar) and in the course of 2 weeks traveled from Lisbon to Santiago, Spain and back, stopping for stays in Porto and Obidos. We were last in Portugal in 2009. In 2012, or so, Portugal began instituting electronic only toll roads - something we had not encountered before. On there roads there is only an overhead reader for the sensor on your car - there are NO booths, manned or unmanned. Europcar (and, I assume, the other car rentals provide a sensor in their cars with a admin fee of about 2 euros/day. You pay the tolls you've incurred when returning the car.
Not all roads are electronic only. Maps of these can be found by 'Googling' electronic tolls Portugal or something similar. If you do encounter one of these tolls and have not activated the sensor, there is a procedure that allows you to pay the toll at a post office of certain tobacco shops after 24 hours and within 5 business days. By the way, there are no electronic only roads in Spain.