The Lost Invitations is a Time Limit Challenge on Floor 5 introduced in Series 4.
The Caretaker's Explanation to the Game:
"In the Tower Post Room, a Grimble's on the loose. It's messed up all the letters, now it's hiding in the booth. What's worse, the Royal Ball is next week, and some of the invitations has gone missing. Unfortunates must find 6 of the missing invitations, stamp them, and post them in the booth. Remember, one of them is not to be trusted."