There was no mention of the dollar on the outside of the envelope, so it is something of a miracle that I opened it! What a wonderful marketing example of the principle of reciprocity (that humans feel an obligation to return a favor when something has been received).
In The Psychology of Persuasion, Dr. Robert Cialdini described the principle of reciprocity with this experiment: Waiters who presented an after dinner mint along with the bill increased their tips an average of 3%. When two mints were given, tips increased by 14%. And, when the waiter left one mint with the bill and returned quickly to offer a second mint, their tips increased by 23%. This clearly demonstrates the power of offering an unexpected gift.(2)