Best way to return to Lisbon Airport from Porto?

By train it's a direct 2:45 to 3:00 hour journey to the city center depending on what departure you take.
By plane it a one hour flight, one-way fares can be hand four around $50usd.
Since it sound like you are just going to the airport and not spending time in the city center, you might as well fly.

But be aware that Porto Airport is the largest airport in the Portugal (bigger than Lisbon's) with lots of flight to lots of destinations so you might try doing an open jaw flight SFO-LIS OPO-SFO.

Thanks, Michael. Good idea to look at Porto Airport to fly back. I checked few flights and most of them have a short 45 min connection in Munich. Any idea about Munich Airport and will 45 min layover be enough?

You're welcome, 45 minutes is cutting it close for a transatlantic flight as there is usually extra security checks and immigration to deal with. 90 minutes would be better.

Thanks, Michael. Appreciate your responses.

Munich is a pretty efficient airport to make a transatlantic connection - much better than Frankfurt for example.

I've done a connection from Florence Italy through Munich to Toronto and I've made it easily in 45 minutes. 90 minutes would give you peace of mind however.

We will be taking the RS Heart of Portugal Tour that begins in Lisbon on about June 13. After the tour is over in Porto we will be flying back to Lisbon on TAP Portugal. We have never flown on them before but could not find any reason not to fly on them. We have a short stop in Lisbon before flying on them back to London where we started this years European travel for a short stay before flying on Delta back to Seattle. Happy Travels

Thanks, Shirley. Hopefully i will find some flight with at least an hour of transit time.

Thanks, Charlie. I'm on the June 20th tour. Have a good trip.