I live in Porto. They recognize US drivers licenses. So you don't need one. That said it's super easy to get one in the US if you want one. Just go to triple A. The prices are cheap but they don't usually include the insurance that is mandatory for foreigners. I highly recommend using a car...
I also fly into the east coast. I recommend flying from Madrid. And try Skiplagged it has great one way options. Also Air Europa has been running specials into Miami.
Portugal has Uber and Volt as well. As others have said on my last camino we traveled with someone disabled and every night the person running the albergue was able to contact a taxi for us. To be fair it was quite expensive in the more remote locations and it obviously took them time to get...
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