Not too familiar with car hire policies ex Faro can I please seek readers guidance.

I have just booked a car for 1 week from April 9th and there is a charge appearing of € 20 for electronic
device presumably for transponder.

Can I avoid this charge by paying my tolls through post office within 48 hrs of usage. What other options

Intend to use A22 toll 6-10 times during my stay. The car hire company is Air Auto from car park 4.

Thanks to contributors to help me on my way in coming week.

If you use the A22 6 times, it will probably cost about €18 so you will have used up € 20. My advice to you would be to use the 125 for any other journeys and you won't incur any extra charges and will not have to concern yourself with any other charges. We use the 125 all the time, it is a more interesting journey from the airport to Carvoeiro and is equally as fast. Enjoy your time in Carvoeiro and I hope the sun shines

Most companies will charge you €20 but that is made up (usually) of €5.00 transponder hire and €15.00 for toll payments - use it or lose it.

So if you use €15 or more in tolls its not such a bad deal. Check on how to top-up when you collect the car.
If you're just using the motorway to and from CVO, then its a rip-off by the rental companies, and they are all in it together

I've never paid Algarvedriver for the transponder. We always use the 125.

We have been using the 125 for the last 3 visits as during the off season it is much quieter.
Undecided what to do for the next visits but will certainky be leaving Carvoeiro earlier to allow for the much heavier traffic.

If you use the motorway 6-10 times as you have indicated then the easiest option is to take the €20 charge from the rental firm as this normally covers rental of the transponder, some charges and their admin.

If you want to avoid all charges use the N125. It is not as bad as many people say and only adds 15 mins to the journey from airport to CVO area. The rental firms refund the €20 if you don't use the motorway - but you should confirm this with your firm.

For just the odd short infreqent hire it is not worth worrying about moe complex options, such as buying your own transponder.

I have used two methods with AlgarveDriver since the tolls came in. First time I declined transponder charge and planned to use just the N125 road free of tolls. However, we were held up on way from airport to apartment by a traffic accident (not us!). This made me late for our rendezvous with the apartment owner who had the key. Took decision to use the A22 to make up for lost time. Two days later I went to Post Office to pay up, but couldn't because they don't have details of cars rented from firms which have the transponder system.
When returning the car I told the rep the story and it appears that if you incur tolls without opting for the transponder, you become liable to pay the full €20 fee. However, because I had used AlgarveDriver before I was only charged €10, which wasn't too bad considering my toll charge was around €6.
Last two times with them I have paid the €20, which is reasonable enabling a quicker journey on the A22 to the apartment and leaving around €10 to use during the holiday. We generally travel back to the airport via the N125, giving ourselves plenty of time for the journey.
So to sum up: it appears you cannot avoid the charge by paying at the Post Office, because they won't have the car from a transponder equipped company on their system.