To take benefit of these prices you will need to prove you are not a business traveller, on the way to the airport, so you will need to give either your flight details or your holiday booking reference. I don't think you will need to provide this when you arrive at redbridge station but you may be asked to provide them on yr travels from the station to the airport.
Going to stansted airport by 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 inexpensive train tickets from redbridge 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 grown up travelling by train to the airport.
make sure you leave enough time to get to stansted airport early enough to eat, shop and so on, so check the times of trains from the station before you go. If you end up with spare time at the airport you can always read or play a game again! As you have bought your ticket in advance, you don't need to worry about the opening times of the station.
Or maybe you meant