As a buyer of a luxury bag, I would be furious if it was shipped folded in a flat rate box. Package it properly - stuffed to keep it's shape, in it's dustbag, wrapped in bubble wrap, then tissue, packing peanuts, or more bubble wrap to fill the box. Make sure your box is large enough, so that the bag doesn't get creased from compression of the box during shipment. There's nothing worse than opening the box to find the bag damaged due to improper packing.