You need not pay Amazon to label your products with your FNSKU to avoid commingling -- sellers can do themselves with a printer and tape. Amazon simply offers the option and charges $0.20/unit to do it.
But if you're already the only legitimate source for that SKU what do you do? If a counterfeiter orders one of your items then duplicates the packaging including the SKU/UPC will it get commingled with your product even if it shouldn't?