I believe they mean that your chicks could arrive as early as Sunday or as late as Wednesday, so plan accordingly. I think you can call them on Monday to make sure but I don't think they will promise you an exact day of their arrival. You may be able to learn from them which postal center they'll be delivered to. Otherwise, contact your local post office. They should know, based on your ZIP code, where your live shipments will be delivered. I like to call my center and let them know that I'm expecting a live shipment, and to make sure they have all my contact numbers to call.
It's fun to get That Phone Call and know what it is before anyone ever says "hello", I can already hear the sounds of my chickies or duckies peeping in the background as soon as I pick up the phone.