Derpy delivers intercontinental(?) mail on a regular basis, not just Ponyvillian mail.
Has competition. Lots of mail to deliver, some do it faster than others.
Perhaps small intro plot about struggles.
Derpy finds some sort of device, perhaps a teleporter, that allows her to deliver said mail very quickly. She doesn't tell anyone about her find.
Potential for subplots regarding mishaps encountered while learning to use it.