Definitely call the post office ahead of time and let them know you're expecting chicks in the mail and ask them to call you when they arrive. It is nerve-wracking, waiting for a package containing live animals, and such delicate creatures at that.
I ordered 4 chicks from them, an EE bantam, a RIR, a Silver-Laced Wyandotte, and a Silver Cuckoo Marans. They arrived today and all are healthy and hale, bright eyes and clean vents. They hatched yesterday, so shipping was very fast. I'm very pleased with them and I'd order from them again.