Ok, I am a very quiet person, I try to keep my mouth shut and mind my own, I try to be polite to everyone... but in this instance, I'd be SERIOUSLY cussing someone's BEE-HIND out.
How dare they?! Don't they realize these are LIVE birds waiting to get to their home and get food and water?! Jee-bus, what demon has taken control of the post office these days?
My sis-in-law had the exact same thing happen to her... only with newborn, 2 day old chicks! Needless to say... by the time the freakin' idjits at the post office figured out where they were and got them to her (with an attitude, may I add, as if it weren't their problem or my sis-in-law as taking up their precious time), the babies were gone.
Hopefully since your Silkies are a bit older, they may also be a bit hardier. I would call and call and call and call until someone answered and helped me out.
PLEASE update us and I'm sure you're in a lot of people's thoughts right now! Here's hoping your little birds make it home safe...