Rain nor Snow nor Gloom of Night

"Avoid obligations," the postman advises.  He stares glumly into a half-full pint of dark amber brew.  "Never make promises."

"Come again?" I ask.

"I swore on my immortal soul that I'd deliver every letter that day.  But there's always pranks, errors, misdeliveries.  I've walked through deserts, over mountains, across oceans... I'm down to three.  Here."  He hands me a letter.  To my great-great-grandson, reads the address.

"What are the others?"

He finishes his drink.  "A blank envelope.  And this."

He tosses it onto the bar.  To: God.

"I'll have some words for Him when I get there," the postman says.