It depends if they have heat packs, food, and water. Usually they don't have the food and water.
Chicks can live up to 72 hours from hatching without food and water, and that's cutting it close if they were shipped Friday. If they were shipped Thursday, I'm sure you'll have losses.
Why would the P.O. get them so late Saturday? I'd really try to be there when they come in as I'd be so afraid some won't make it. Shipping is hard on them as it is.
I would also check with your P.O. about this as they are known to make exceptions for people with live chicks. They made one for me for just hatching eggs.
I'd definitely drop by Saturday and stay until they arrive. The sooner you get those chicks in your brooder the better. Definitely talk to your postal workers as well so they know what's up