Oddly enough, Florida has no laws regarding the timing of your last paycheck; nor is there any law requiring that it be mailed. Common law says that you must receive it by the next regular payday but there is no statutory support for this in Florida. Florida has no laws at all regarding the method by which it must be sent.
I'm not aware of any law that prohibits them from requiring that it be sent registered. Florida has no department of labor; you are free to ask the US DOL if they have any laws prohibiting that you be charged the registration fee.