No it is not legal. She can impose one late fee if it is more than five days late. If you pay monthly it cannot exceed 5% of the rent so you would owe $62.50.

Refer her to NC General Statute 42-46. The lease cannot override this provision.

(a) In all residential rental agreements in which a definite time for the payment of the rent is fixed, the parties may agree to a late fee not inconsistent with the provisions of this subsection, to be chargeable only if any rental payment is five days or more late. If the rent:

(1) Is due in monthly installments, a landlord may charge a late fee not to exceed fifteen dollars ($15.00) or five percent (5%) of the monthly rent, whichever is greater.

(2) Is due in weekly installments, a landlord may charge a late fee not to exceed four dollars ($4.00) or five percent (5%) of the weekly rent, whichever is greater.

(3) Repealed by Session Laws 2009-279, s. 4, effective October 1, 2009, and applicable to leases entered into on or after that date.

(b) A late fee under subsection (a) of this section may be imposed only one time for each late rental payment. A late fee for a specific late rental payment may not be deducted from a subsequent rental payment so as to cause the subsequent rental payment to be in default.