The Post Office is directly to the right of Nim the Translator (the first goal given to you by Posteradactyl).
You will see a locked bumper on the ground and a green Tucan/Parrot-like bird above. Pay 30 fruits to unlock the bumper, use it, and once you land in the post office you should unlock the achievement.
*** Spoiler - click to reveal ***Talking to the Tucan/listening to the recording will also grant you the Mailbag which allows you to randomly stuff letters into Mailboxes to gain more fruit.