"Why, hello, Twilight! What brings you into our fine postal establishment?" Derpy asked, consigned to desk duty for a week after her latest "incident".
Hello, Derpy. I have a package I need to jì."
"You've certainly come to the right place! What is it that you'd like to jì?"
Twilight reached into her mane with her magic and pulled out a long pipe. And pulled, and pulled, and pulled... in a feat of hammerspace that would make Pinkie jealous, Twilight managed to produce ten feet of metal piping from within her hair. Derpy stared, concerned and fascinated. "Uhhhhh..."
"Pretty cool trick, right? Anyways I just need to jì this quantum thermo deoscillator to my human counterpart at Crystal Prep high school on the other end of the mirror. It won't go through for some reason.
"Princess, permission to speak freely?"
"Always, Derpy."
"You couldn't get this, uh, thing through your mirror portal thingie, right?"
"Exactly. It kept spitting the apparatus out."
"So how do you think the Equestrian Postal Service is supposed to jì it?"
"Uh... magic?" Twilight responded hopefully.
"We're letter carriers, not miracle workers. If you want inter-dimensional transit, talk to Pinkie Pie. Apparently she has some connection named Ramona Flowers who she swears by."