Buy a TAG - Eurotunnel resell the French Emovis one that can be enabled for Spain & Portugal. It is expensive compared to the APRR one I mention above. With APRR you only pay a fee in a month you use the TAG. Emovis charge an admin fee every month which is not good if you only use it twice a year or not at all this year.
As I state above, on the couple of short motorway sections I did near Porto, there were not any toll barriers. You ride through a gantry that reads your number plate. In France & Spain there are barriers which only go up if it recognises the TAG or you use the alternative "take ticket as you enter, pay with credit card on exit" system.
https://www.eurotunnel.com/uk/promotions/emovis-tag/ - you need to be a Eurotunnel customer to get the advertised discount
http://www.aprr.fr/fr/telepeagelibert/particuliers/offre-topeurop I use this system. There is a one off 15 euro set up fee then you just pay an admin fee of 2.4€ in any month you use it, plus the tolls of course. Website is in French only by Chrome translates. I have set up an auto payment of bill using my Post Office credit card which does not charge foreign currency commission.
If you subscribe to any of the TAG systems, makes sure you have the correct vehicle registered on the system. I find the APRR online system easy to navigate using Chrome to translate or my rudimentary French.
I don't think I have added any new information that is not already in the thread above.