The Colony was under blockade by hostile alien life, thereby making it impossible for him to deliver the mail. Okay. If he'd been a regular member of the Postal Service, he'd have given it up as a bad job and gone back to Earth to report the difficulty.
But I'm not going back. I'll be the best damned mailman they've got.
"Give me a landing orbit anyway, Ganymede."
"But you can't come down! How will you leave your ship?"
"Don't worry about that," Preston said calmly.
"We have to worry! We don't dare open the Dome, with those creatures outside. You can't come down, Postal Ship."
"You want your mail or don't you?"
The colonist paused. "Well—"
"Okay, then," Preston said. "Shut up and give me landing coordinates!"