Right,
To begin with as a resident in Lisbon who works in Marbella, I travel this journey about twice a week.
In Portugal the A22 in the Algarve requires you to pay the toll only using the electronic "Via Verde" tag. As I own a Spanish registered car, I have not paid any tolls in Portugal since 2010, just like all the Spanish tourists do every time they visit Portugal. You can simply pass through the "Via Verde" lane on all the motorways where there are normal toll booths, as it does not have a barrier.
So to put it simply, if you have a foreign registered car, they will not charge it or catch you for it (see below.)
This past December I decided to get myself a Via Verde and to do it honestly. Upon signing up for the Via Verde at the office, I confessed to them of what I have been doing and volunteered to settle the account on the spot. The employee searched for my registration plate, and there were countless photos of my car passing through with a balance of 600+ Euros, and we were both laughing about the matter.
So, she got her manager to try and help her settle the account. Her manager informed me that they have no mechanism to collect voluntary payment for these outstanding bills, and she simply told me that since I have a Spanish car it is impossible to fine me for that as they can't trace it, so she just told me to forget about the whole situation.
So, there is my recommendation. Just drive as you normally would. Portugal has much more pressing bureaucratic issues to deal with than going through the court system of Spain or another country to track down your information and send you a fine for 10 Euros of unpaid tolls! It just is the way it works over there.
On the other hand, the motorway on the Spanish side from Seville to the Portuguese border is free, and if you want to take a free road, you can cross the border at Ayamonte and take the N-125 road across the Algarve.