We drove from Madrid to Lisbon. No electronic toll roads. Will be heading to Algarve and, if we want to use toll road, will need pass because they are electronic. From the toll website, seems our options are limited. What are best options? Are they really at post offices? Haven't seen any. We know we can't use transponder as we return car in Madrid.
The main toll roads have lanes to pay by cash, as well as the lanes for the electronic transponder. A2 is the toll road between Lisbon and the Algarve and will have cash lanes. Just avoid the Via Verde lanes.
Once you get to the Algarve, it's pretty easy to avoid the toll roads. And, the non toll roads in the Algarve are quite adequate if you aren't trying to travel very long distances. But the A 22 in the Algarve does not have cash lanes, so if you want to use it, you will have to go to a pay shop.
I spent two weeks in Lagos without using the toll roads and drove around a lot. So, you can get by without using toll roads if you want.
We drove from Seville to Lagos.
We stopped at the first service area in Portugal to buy our card for the transponder. We saw no toll booths.
Once we bought it (the woman there told us how much we needed, I think it was 10 €. We had to text a code from the ticket to a number on the ticket. A minute or so later, we got a text back and took off.