To take benefit of these prices you will need to prove you are a leisure traveller, on the way to the airport, so you will need to provide either your flight numbers or yr holiday booking reference. I don't think you will need to provide this when you arrive at penryn-cornwall station but you may be asked to give them on yr journey from the station to stansted airport.
Going to the airport on the train is great, you can take a nap, read (even at night), play travel scrabble, or just gaze out of the window... The other caveat to buying these inexpensive 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 5 go free, and then one kidover 5 but under sixteen per adult travelling on the train to stansted airport.
make sure you leave enough time to get to the airport early enough to eat, shop and so on, so check the times of trains from the station before you go. If you have spare time at the airport you can always read a book or play travel scrabble again! As you have bought yr ticket in advance, you don't need to worry about the opening hours of the station.
Or maybe you meant