Oh Heck No! In fact, most hatcheries recommend that you let your PO know, up front, that your chicks will be arriving in a day or so, so they know to hold them for you. There should be no extra charge for saving the carrier some work and saving the desk clerk from all the paperwork they'll have to fill out when you have to claim your dead chicks as a loss ... at the PO's expense!
Edit to add: It sounds like someone is trying to sponge a few extra bucks off of you. Talk to the Postal Manager, right away!