Not a very constructive answer but I used to live in Lisbon and car rental struck me as incredibly economical. A woman started a business there putting racks in the hotels with business cards / tiny flyers in them. The independent car rental companies with their cards in the racks have stunning rates. Or at least they used to. I used to get a little Fiat if I was bored at the weekends just because driving around the cobbled hills in Lisbon is hilarious fun. Don't know Pinhao but the roads to Coimbra and Porto are great quality

I am sure they could fix you up with an auto for a bit more.

In short, don't book online, book an indie company when you arrive from the little racks in the hotels