Florida law recognizes a concept of "usury"
All contracts for the payment of interest upon any loan, advance of
money, line of credit, or forbearance to enforce the collection of any
debt, or upon any obligation whatever, at a higher rate of interest
than the equivalent of 18 percent per annum simple interest are hereby
Late fees are liquidates damages, not interest on a loan, so this law is irrelevant. Violation of the prohibition against usury is criminally prosecutable, under §687.146.
Instead, late fees have a separate potential limit under §83.808, which declares that a landlord "may charge a tenant a reasonable late fee for each period that he or she does not pay rent due under the rental agreement", and also declares that "a late fee of $20, or 20 percent of the monthly rent, whichever is greater, is reasonable and does not constitute a penalty", plus it allows collecting reasonable enforcement expenses, which are not defined. The law does not prohibit late fees greater than 20% or declare them to be penalties, it simply does not statutorily declare them to be reasonable, in which case the court must assess whether the fee is reasonable. The landlord would have to shoulder a substantial burden to prove that a fee above 20% is reasonable. Assuming, for example, a 25% late fee (and not including separate collection fees), and assuming that the landlord failed to convince the court that the fee is in fact reasonable, the courts would limit the fee to what it deems to be reasonable, and there cannot be criminal prosecution of the landlord for having such a clause in the lease.