We have a system where we Use Dinky as an intermediary. The Filly has an uncannily good sense at figuring out where her mom is, be that if she's working at the Post Office near the Train Station, or where she is if Derpy's working her route. That includes Dinkster figuring out when she's about to bomb my couch through the window. It's not always the same time, either. I'm not going to pretend like I know how the mail system works in Ponyville, the packages get around either way.