Hi,
There is a bus leaving Valenca at 13:15 (Portuguese Time) that drops directly at the airport. Only runs once a day though. Find it on
international area of alsa.es
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There are four other buses on that day (25th) that I can find that run to Porto City and you could then get the metro out to the airport for your flight.
http://www.rede-expressos.pt/
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Lastly, there are many trains a day, some requiring changes. Look at
Http://cp.pt and search Valenca to Porto - Sao Bento
View attachment 18559
Timetable for Autna bus mentioned by
@whariwharangi above.
View attachment 18587
All seem that you can book on-line.
Scenery wise the train runs along the coast for a short stretch, the airport bus runs inland via Braga, the other four buses appear to stop in a few coastal towns.
I hope that helps.
Buen Camino.
Mig