(didn't read thread, apologies)
i feel ya on asking girls out at their job, you gotta be cool about it to not come off douchey or creepy.
but the post office setting offers exciting possibilities! you could D.E.N.N.I.S. her and go in sending stuff to your sick auntie or something if that's your kind of thing, you could just send large empty boxes a couple times til she asks wtf youre doing...
or, more conservatively, you could just actually send stuff a couple more times and if you get a chance, tell her you'd like to see her again
as long as you don't come off too aggressive or weird, the worst case scenario is she says no, and even then in most cases she'll be flattered (this entire post assumes youre a reasonably chill dude). either way, do it dude, i guarantee you won't regret it..even if she says no, you'll be happy you tried and you'll learn from it