If you choose USPS priority mail, your order will arrive in a USPS white priority box (or if perhaps we've run out of a certain size of brown boxes). The majority of the companies out there use either a brown, or a white box for their shipments.
I've had both the brown boxes and items sent in Priority boxes from the post office. I suspect the company uses the Priority boxes because the post office will provide those free - in bulk - to business that ship via Priority mail. I'm sure smaller items that are quite heavy (like steel toys, for instance) would ship at a more reasonable price in a small "flat rate" Priority box, too.