He opens it and discovers it is from an elderly lady, distressed because some thief robbed her of 100 dollars. She will be cold and hungry for the rest of the month if she doesn't receive some divine intervention.

The worker organizes a collection amongst the other postal workers, who dig deep and come up with 96 dollars. They get it to her by special courier the same morning.

A week later, the same postal worker recognizes the same hand on another envelope. He opens it and reads:
"Dear God, Thank you for the 100 dollars. This month would have been so bleak otherwise.
"P.S. It was four dollars short but that was probably those thieving bastards at the Post Office."