The first time is indeed anxious. But you did the right thing to contact your local post office and give them the heads up that you're expecting them and will make your pick up as soon as they notify you.
Your contact # is probably on the shipping label but it doesn't hurt one bit to give it to them again. And, as my very urban post office in Los Angeles says, "You know, it's NOT the first shipment of chicks we've handled!
If it's HOT there in Louisiana, it's not a bad idea to show up at the post office with some Nutridrench or some electrolytes. ...just in case. But, until you hear from the PO, relax a little and just be ready to go collect them when you get the call.
When you do pick them up they'll hand you a cheeping box and the whole room will melt. It's really kinda fun to see the big hulking guys go all mooney over baby chicks.