To take advantage of the best prices you will need to prove you are not a business traveller, en route to the airport, so you will need to give either yr flight details or your holiday booking reference. I don't think you will need to give this when you arrive at penryn-cornwall station but you may be asked to give them on your travels from the station to the airport.
Going to the airport on the train is great, you can sleep, read a book (even at night), play travel scrabble, or just gaze out of the window... The other restriction to buying these cheap train tickets from the station to stansted airport is that there must be at least two adults travelling together. But another great advantage is that children go free. All children under five go free, and then one child over five but under 16 per grown up travelling by train to stansted airport.
be sure to leave enough time to get to the airport early enough to eat, shop etc, so check the times of trains from the station before you go. If you end up with time to kill at stansted airport you can always read a book or play travel scrabble again! As you have bought yr train ticket in advance, you don't need to worry about the opening hours of penryn-cornwall station.
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