Welcome to BackYard Chickens. I'm very sorry to hear about your chick.
Would you mind posting a few photos of her, or a video?
Especially with chicks that have been shipped across country, on occasions due to a lengthy shipping time they can suffer from hypoglycemia (low blood sugar) which can result in seizures. Other cause's for a chick this age to have seizures, may include nutritional deficiencies such as in thiamine (B1) or vitamin E, infection from an unabsorbed yolk sac, or trauma to the head (common with silkies).
For birds having seizures, you really need to move them into a warm, dimly lit, stressful free, padded area. I would not have water in the pen as she may aspirate, or drown on the fluids. Instead, periodically offer her water often.
Heavy breathing is not a good sign, and likely suggests she is in a very debilitated condition. If she was mine, I would try, very carefully, dripping a drop of a poultry vitamin on her beak, or drip a high concentrated electrolyte or sugar water. Baby birds aspirate on fluids very easily, as they still don't have the whole "how to drink" process down well, so be very careful administering fluids. Besides that, keep her warm, offer mashed scrambled eggs if you can wait. If she doesn't improve, or her condition worsens it may be best to euthanize.