In another Portugal post, a forum contributor said, "If you rent (a car) in Spain check out the portugaltolls website (https://www.portugaltolls.com/en#sistemas-portagem). It provides everything you need to know for paying tolls with a foreign registered vehicle."
Since we're driving a rental car through Portugal from Santiago de Compostela to Sevilla, I checked out the site... Good information there. It doesn't appear to difficult or expensive. I also checked Google Maps to see if we could avoid the toll roads entirely, which we can apparently... I wonder, though, how much more (if at all) desirable the non-toll roads may be. It appears via Google Maps that non-toll roads take cars mostly along the coast. To me coastal drives always are more appealing (unless one is driving along the Pacific Coast Highway in Southern California on a summer holiday!).
Avoiding Portuguese toll roads will add time in the car, but that seems to be the only downside... And that is offset by driving on those less traveled roads -- seemingly along the coast.
Thoughts?