A mailman notices a mailbox with the flag up.
So, he opens the box and picks up the letter. He glances at it briefly to make sure it is stamped, and then puts it in his bag with his other letters. When he gets back to his office, the letter goes in a big bin with all of the other out-going mail. He thinks nothing of it, and finishes his day.
A few days later, he delivers the exact same letter to the same house that he picked it up from. He glances at the mailing address and observes that it is indeed the same as the return address. Anyone can make a mistake, so the mailman puts the letter in the mailbox so that the customer can readdress it for its proper recipient. He thinks nothing of it and finishes his day.
The next day, the mailman sees the same mailbox with the flag up. He opens the box and again sees the same letter, nothing changed, but with a new stamp on it. The mailman is perplexed, and thinking to save the customer both time and money, decides to ring the doorbell and inquire about the letter.
Ding-dong, ding-dong. Ding-dong, ding-dong. Ding-dong, Ding-dong.
The door opens and standing there is a stout Buddhist monk, dressed in traditional garb.
“Pardon me, sir,” the mailman says, “but you seem to be trying to mail this letter again, and without any changes to the address, it’s only going to end up back at your home in a few days.”
“Ah, my letter. Thank you so much for mailing it the other day, it was greatly appreciated. Please do so again” replies the Buddhist monk.
“But sir,” says mailman, “you will only waste a stamp, and this letter will be re-delivered to your home a few days from now.”
“But that is my intention, dear man,” replies the Buddhist monk. “You see, reposting is the best way to get karma.”