I've done this before with things like displaying an image saying "press E to open" when you go close enough to a door..
What would work is that you make a trigger box around the letter for the area where the player can press I to read
in the first state it has a "trigger event" event waiting for the player to walk into the box - when player is in the box it will flow to state 2
in state 2 you use the "get key" action which will wait for a key to be pressed
When the key is pressed (in this case "i" ) then it will flow the next stage where you will have it set the alpha of the gui texture to 100 so its 100% visible, and listen for "i" key again
when "i" is pressed again it will flow back to the 2nd stage - setting the alpha to 0 (making it invisible) and ready for the user to press "i" and open the letter again
in both stage 2 and 3 you need to have a trigger event action that listens if the player leaves the letter area.. so if player leaves trigger then flow it back to stage 1 where it listens for the player to be in range again
to make this work you of course have to add the GUI texture to the scene first and give it a name and place it where you want it etc - and then when you start the game the letter will flow into its first state with the listener and setting alpha to 0, waiting for the player to be close enough and pressing "i" to flow into 3nd state which sets alpha to 100 and listens for "i" again to close the letter
Hope it's explained well enough - if you have any questions, feel free to ask