It looks like we could easily take a train plus taxi from Porto to the Lisbon Airport in about three hours time. I've read that there are many trains each day on that route. I would like confirmation from any of you seasoned travelers before I make my plane reservations from US. Thanks in advance.
Check Skyscanner. I've seen non-stop flights (Porto to Lisboa) for under $50. Be certain you understand the luggage restrictions - sometimes the prices are "too good to be true".
I can confirm that it's true. Train travel in Portugal is affordable, and advance "promo" tickets start at less than 10€ for some departures. Check the timetable and book your tickets on https://www.cp.pt/passageiros/en
You can also take the metro from Lisbon Oriente Station to Lisbon Airport - it's a short ride.
I just did that same trip (but by air) yesterday. Our RS guide for our tour always takes the 3 hr train from Porto to her home, Lisbon. The folks I was traveling with wanted to fly instead so that's what we did. Of course the journey was shorter (40 minutes) and the price very reasonable but I wish we had taken the train to see more of beautiful Portugal.
From campanha in Porto there are alpha pendular trains to Santa Apolonia or Oriente in Lisbon. Allow 3 hr although trip is slightly under 3. Several trips each day. Then you can: metro to LIS or taxi or Uber. Online tickets from Campanha are only through the official website not Rail Europe. I bought tickets about 2 days before our trip enroute to Porto. We flew back to US from LIS.
I would take the metro from Lisboa Oriente to the airport. It's fast and easy. Taking a taxi or uber will be more of a hassle and probably take more time because of traffic.