Hidden fees are annoying, you end up paying way more than the initial quote showed. Local providers tend to be more flexible with pick-up and drop-off arrangements, plus they usually know the area better and can give you actual useful tips about parking and routes. You can check where to rent a car in Oeiras from local owners here
https://getrentacar.com/en-US/portugal/oeiras . They've got various models available and the rates are typically more reasonable than the big chains. You can arrange to pick up the car right in Oeiras near your conference venue, which saves you the hassle of travelling extra distance just to collect your wheels.